Improving fuel economy

[masospaghetti]

I've been a happy owner of my 2006 Envoy (2WD, 4.2, SWB) for about 5 years now. My main gripe is that the fuel economy is not very good. My previous vehicle was a 2001 Tahoe w/ 5.3. The Envoy reliably gets about 1 MPG better than the Tahoe, ~15 mpg around town and ~23 for freeway driving.

Obviously this is going to vary a lot by driver and vehicle condition, so let me say that I drive pretty conservatively and the vehicle is well maintained (tire pressure, fluids, plugs, etc). I use synthetic motor oil, Dexron VI in the transmission, and synthetic gear lube in the differential.

I am looking for suggestions to improve fuel economy that are beyond just good maintenance. Any room for improvement via:

1. Computer tune? maybe make the TCC lock up more quickly? (the Tahoe would lock the TCC almost immediately after acceleration, the Envoy has a significant delay)
2. Aero improvements, such as larger air dams in the front? (You can see this in newer trucks, they often have huge lower air dams)
3. Anything else I haven't considered?

I know this is never going to be a Prius but any improvement is welcome. Regular unleaded is over $5 now here in CA.

(As an aside, it seems odd that the Envoy w/ 5.3 gets better gas mileage than the 4.2, at least from the EPA)

 

[littleblazer]

The 5.3 gets its better rating with AFM... in reality they're about equal.

The easiest way would be a regear to the 3.42 ratio if you don't already have it.

Narrower tires would help a little as well as using low rolling resistance ones.

You could do a 6 speed swap... that would help around town immensely with the regear and still provide good get up and go.

Book some time in a wind tunnel to see what areo can be improved.

You could have a lean cruise tune made perhaps... at steady state rpms the AFR goes leaner. This isn't great for emissions but helps fuel economy.

Off the top of my head, that's about all I can think about.

 

[azswiss]

A minor opportunity: remove the roof rack cross-pieces. Antenna??

 

[Mooseman]

It's a recurring subject that never really gains any real results. These trucks were never really fuel efficient because they are trucks. People seem to think because it's "just" a 6 cylinder it should be more efficient. This is still a 5000lbs brick. And now with increasing fuel prices thanks to that crazy Russian, it's gonna hurt us all.

 

[Matt]

The best way to improve the MPG's is to buy a smaller 4 cylinder and leave the truck in the driveway.

 

[c good]

The best way to improve the MPG's is to buy a smaller 4 cylinder and leave the truck in the driveway.

This is what I've done. Bought a 1997 Toyota Corolla, 5 speed manual trans. (35 mph) It's my daily driver. Envoy is just for weekends.

 

[Mooseman]

I actually went bigger and get better fuel economy. 2021 Sierra with the diesel I6 3.0. I've gotten north of 25mpg unloaded. Some have reported 30mpg.

 

[Matt]

You all know what I did, got 26.5mpg this morning putting the slipper into it a bit.

 

[BigJ]

7 Hypermiling Tips to Help Improve Your Fuel Economy

Under-inflated tires and extra weight in your car can reduce your car’s fuel economy. Learn how you might save money at the pump by hypermiling.

blog.nationwide.com

 

[BigJ]

7 Hypermiling Tips to Help Improve Your Fuel Economy

Under-inflated tires and extra weight in your car can reduce your car’s fuel economy. Learn how you might save money at the pump by hypermiling.

blog.nationwide.com

when I drive my wifes 07 Tahoe 5.3L I can get 20mpg, takes time, but its reachable, her? maybe 15mpg. I drove OTR truck and was a Owner operator. I paid for my fuel cash out of pocket, so maybe that had something to do with it?. on the fly, with wht one has already, avg 3-5mpg can be gained just by changing your driving habbits. My current daily driver for commuting 05 BMWX5 3.0L gasser, 22-24Mpg, 98% is hwy driving though. I am al so looking at a replacement for the x5. Not a huge fan of Direct injection, but with 4 banggers getting 40,60 and possable 70mpg on a small direct injected/turbo unit.... I have to take a step back...

 

[masospaghetti]

Thanks for the suggestions so far. Good suggestion about the roof rack cross bars!

Is the 6-speed auto swap really a thing for these vehicles? I wouldn't do it just for the MPG's, but maybe if the trans was on it's way out.

 

[Mooseman]

Is the 6-speed auto swap really a thing for these vehicles? I wouldn't do it just for the MPG's, but maybe if the trans was on it's way out.

I don't think so. Never heard of it done on this platform and definitely not on the I6, which doesn't share a bellhousing with any V8 engine. And forget about getting the electronics to work. The 6L80E is completely electronically controlled with the TCM built in and not even in the same generation as the I6 controls.

 

[TequilaWarrior]

This is a perennial subject (fuel economy).

The biggest difference will be driving habit (lead feet use more gas).
Second biggest is terrain... (hillier area, worse fuel economy).
Removing roof racks will help a nearly immeasurably small amount.
REDUCING WEIGHT is the next improvement (still smaller gains). Reductions are kinda percentage-based and these weigh over 5000 lbs to start. Dropping 25lbs is a drop in the bucket.
Air damming is virtually pointless on this size/shape vehicle - it's like a smaller UPS van ffs. Although some have said the "bugshield" that's available for these does help a little.
Tunes for economy are also virtually pointless as any economy gains are going to be hard to achieve consistently while keeping the vehicle driveable - our PCMs aren't flexible enough to be both efficient and powerful and know when to do which. From the factory our PCMs adapt to several factors, but there's no knob to turn to choose between "power", "economy", "towing". It's always set to "balanced".... well, tbh, it's always set to "torque", but that's a different discussion to have altogether.

Anecdotally, I remember other GMT owners reducing weight (remove spare tire and spare tire carrier, remove rear resonator, one guy removed all unused seats), removing crossbars (which I've done, but replaced it with my lightbar so, no real difference). But, consistently, the only thing that gives good results is driving habit.

On a normal day, I'm lucky if I break 13 MPG on the upside but I drive it like I stole it 5 minutes ago and where I live is extremely hilly. When I had a nearly 300 mile commute to my MA office (years ago), I drove as economically as I could to get the whole commute in 1 tank and usually did... that was just over 18 ish MPG. 5 MPG, just by driving habit and the fact that I was on a long straight stretch of highway for most of it. Unfortunately, there's nothing you can "bolt on" to get that kind of difference.

 

[littleblazer]

I don't think so. Never heard of it done on this platform and definitely not on the I6, which doesn't share a bellhousing with any V8 engine. And forget about getting the electronics to work. The 6L80E is completely electronically controlled with the TCM built in and not even in the same generation as the I6 controls.

I recall someone starting it and having it working... there were a couple SS guys running it. They make stand alone setups that work with the 6 speed. Is it more money than its worth? Hell yes. But not impossible to accomplish. For the half a MPG it may gain it isn't worth it for sure but it can be done...

 

[Mektek]

The improvements are incremental.
Fairings for the rear wheel openings make a small aerodynamic improvement.
There are Raybestos drag reduction clips available for the front calipers. I am experimenting on the MC with stronger springs that actually retract the caliper pistons and almost eliminate parasitic brake drag. The experiment is still in progress........

 

[Reprise]

Not much more to say here. @TequilaWarrior has good advice above.
Longer trips give more potential for higher mpg (e.g.; a couple hours or longer on the interstate). Every time you have to stop and start again, that kills your average. I've gotten 26mpg with my Envoy XL on a long interstate trip once, but my around-town average with it is more like 16.

Your driving habits are the biggest factor in the MPG you'll get. But in a heavy vehicle like the GMT360, with the rough aerodynamics of a brick... you're probably talking about 3mpg or so. Just not a lot of room for potential improvement, unless you decide to use some of the more radical hypermiling techniques (I only mention this because there are people willing to do things I myself won't, in order to save gas; the info is out there, if you care to look.)

One other thing you can do is decrease the rolling resistance. If you're running A/T tires, think about a new set with a highway tread. Square shouldered tires are good for mud, not so great for MPG. Same for ultra-wide tires.

The least rolling resistance will come if you can tolerate inflating to max sidewall pressure, as indicated on the tire. But your ride quality will suffer. I can tolerate a good 5-6 lb above the stated pressure in the door placard, and your tires will wear a tiny bit better at that pressure, as well. If you can tolerate somewhere between 5 lb and max sidewall greater, that will only help.

More and more tires are being made with low rolling resistance (LRR) compounds. There are some that advertise this (I don't know of specific truck tires, but I'm sure there's something out there). I had LRR tires (Bridgestone Ecopia) on a Civic Hybrid some years back, and I hated them in the wet (so did Honda, but that's another story). I had a nice paragraph written about having to get dedicated snow tires, but I see you live in Cali, so that's not really a concern for you.

So... if you're shopping for new tires, and a set of LRR tires appeal to you... make sure wet traction will be sufficient. Obviously, an 'A' rating on the sidewall is something to look for... but also look for others' feedback on the tire you're interested in (Tire Rack has excellent feedback / reviews; I look at them when I'm in the market for new shoes for one of the 'kids'.)

If you have a more fuel-efficient vehicle to drive, and you're sensitive to gas prices, use it. Save the truck for occasions where you need it, while prices are spiking.

Finally, what you *pay* for gas can be addressed, if only in a small way. If you have a Costco nearby with a gas station, they tend to have some of the cheapest gas (and it's top-tier rated, believe it or not.) If you don't already have a membership, the cost of that has to be figured in, of course. But if you buy a lot of gas, it can add up.

Over here, I use Gas Buddy, both to find the best prices nearby, and to get discounts (which stack on any other discount programs you have). Example -- I use Shell's 'Fuel Rewards' program which gets me .05 / gal (sometimes more, but always at least that much). So between the two of those, I always save .10 / gal minimum, when I use Shell. There's another regional supplier that gives me .03 -- .06 / gal, and I use them, too. When I'm towing the camper, I get discounts from programs linked to that activity (and at about 8mpg, I need all the help I can get!)

With the Gas Buddy program ('Gas Rewards' -?), it's registered to a checking account, and the funds are withdrawn a couple of days later (their discount is applied when the funds are withdrawn). It's a 'fleet' card program (Wex, IIRC), and the one major brand that doesn't participate is Exxon / Mobil (I know they have a big presence on the West Coast, so that might be a factor for you).

I used to use my AMEX for gas, because they gave out 4x rewards points on groceries / gas. But when they stopped doing that on gas, I stopped using them and signed up with Gas Buddy.

 

[cornchip]

I don't think so. Never heard of it done on this platform and definitely not on the I6, which doesn't share a bellhousing with any V8 engine. And forget about getting the electronics to work. The 6L80E is completely electronically controlled with the TCM built in and not even in the same generation as the I6 controls.

True Blue Motorsport still has this on their website. It's a lot of coin for what effectively the same first gear (4l60) and a slightly deeper OD. The real mileage magic starts with the 8 and 10 speeds that get the mass of the vehicle moving off the line with even lower and better spaced gearsets. My JL Wrangler with the ZF inspired 850RE does a fine job of this with a noticeable gain of 2.5L/100 km's for driving over the TB I had. So yeah.....keep those RPM's down ↘️

 

[Mooseman]

The 6L80E has a deeper 1st gear ratio of 4.03. The 4L60E has a 3.059. This allowed GM to put taller gears in the diff with the 6 speed. I noticed that with the 6 speed they kept 3.73 or 3.42 gears so the final drive with overdrive of the 6 speed was even better keeping RPMs even lower. The final overdrive of both trannys are essentially the same at .696 for the 4L60E and .67 for the 6L80E. Higher HP (SS) and higher towing capacity trucks were outfitted with 4.10 gears with the 4 speed. I had that in the Avalanche and MPG were brutal in that truck with the 6.0L.

Hell, I have 2.92 gears in my Caprice with the 6 speed and 6.0L. I takes off like a bat out of hell but still manages 17MPG avg and a best of 22MPG.

 

[cornchip]

The 6L80E has a deeper 1st gear ratio of 4.03. The 4L60E has a 3.059. This allowed GM to put taller gears in the diff with the 6 speed. I noticed that with the 6 speed they kept 3.73 or 3.42 gears so the final drive with overdrive of the 6 speed was even better keeping RPMs even lower. The final overdrive of both trannys are essentially the same at .696 for the 4L60E and .67 for the 6L80E. Higher HP (SS) and higher towing capacity trucks were outfitted with 4.10 gears with the 4 speed. I had that in the Avalanche and MPG were brutal in that truck with the 6.0L.

Hell, I have 2.92 gears in my Caprice with the 6 speed and 6.0L. I takes off like a bat out of hell but still manages 17MPG avg and a best of 22MPG.

Ouch.....I didn't know the 4l60 was that numerically high. I would have thought more like 4 or so. That's a real stinker.

 

[MD_Envoy_Driver]

Draft semi trailer trucks? I used to do this in a Scion Xb and was amazed at the improvement of my MPG. I don't recommend it for the long-haul, but I have about 16 total miles of freeway in my commute and tuck in behind a semi when possible. Yeah, I know.....somewhat dangerous and possibly illegal depending on how close you get to the back end.....

 

[Mooseman]

And may get a brake check if the driver finds out because it creates extra drag for them.

 

[flyboy2610]

Draft semi trailer trucks?

DO NOT EVER DRAFT A SEMI!!!!
He cannot see you in his mirrors, and you don't know what's happening on the road in front of him! If he has to hit the brakes suddenly you are going to wind up under his trailer with your head ripped off!
If you want to get yourself killed due to your own stupidity, that don't matter to me. But you're liable to get someone around you killed as well.
That does.
You may be thinking "Geez, man, lighten up!" My current job is a die setter/press operator. Before that I drove 18-wheelers for a living, both long haul and regionally. Why I got out of it is not a discussion for here.
But I have seen first hand the results of accidents where idiots have been drafting semi's. The carnage from some of them has been unbelievable! I have had truck driver friends get seriously injured because of those people! Innocent people in vehicles around the idiot have died because of drafters.
Maybe I am a bit over-the-top, but after what I've seen I can't stress it enough:
DO NOT EVER DRAFT A SEMI!!!!
We now return you to your regularly scheduled thread.

 

[vipergg]

I've been a happy owner of my 2006 Envoy (2WD, 4.2, SWB) for about 5 years now. My main gripe is that the fuel economy is not very good. My previous vehicle was a 2001 Tahoe w/ 5.3. The Envoy reliably gets about 1 MPG better than the Tahoe, ~15 mpg around town and ~23 for freeway driving.

Obviously this is going to vary a lot by driver and vehicle condition, so let me say that I drive pretty conservatively and the vehicle is well maintained (tire pressure, fluids, plugs, etc). I use synthetic motor oil, Dexron VI in the transmission, and synthetic gear lube in the differential.

I am looking for suggestions to improve fuel economy that are beyond just good maintenance. Any room for improvement via:

1. Computer tune? maybe make the TCC lock up more quickly? (the Tahoe would lock the TCC almost immediately after acceleration, the Envoy has a significant delay)
2. Aero improvements, such as larger air dams in the front? (You can see this in newer trucks, they often have huge lower air dams)
3. Anything else I haven't considered?

I know this is never going to be a Prius but any improvement is welcome. Regular unleaded is over $5 now here in CA.

(As an aside, it seems odd that the Envoy w/ 5.3 gets better gas mileage than the 4.2, at least from the EPA)

Have a 2008 Izuzu Ascender 2WD and that is about what I get . you are never going to get more than about 23 mpg highway . The main drawback is they used 4 speed autos back then not the 8 and 9 speeds of today which would probably get you up to like 25 mpg if had a modern transmission .

 

[masospaghetti]

Thanks everyone for the input! It sounds like the 6-speed is not a realistic option and is way too much of a fab job than I have the appetite for.

I have considered a smaller vehicle and will probably get an EV if/when prices come down to Earth. Rav4 Primes are pretty compelling but they are almost impossible to get at a reasonable price.

I like the idea of tuning or using lean burn, but after checking around, they probably are too expensive and would not pay for themselves. HPTuners is $399+, maybe there are others.

I've been experimenting with improvement with a few different ways:

1. As suggested, removing the roof rack cross bars. This is easy and only takes a few minutes.

2. Adding a seal to fill the gap between the grill and bumper. This is a simple "z" shape adhesive seal that is also easy to install. I personally thought the gap also looked bad and so this is a win-win. Here is the seal installed. I probably should have used a slightly larger seal to fill the gap better.



3. Removing the spare. For around-town driving, the spare may not be necessary (especially for folks with AAA coverage). Also, if your spare tire is the original, would you trust it anyway? It's fairly heavy. If it's 60 lbs that would yield a 1%-1.5% improvement in around-town economy if we figure a vehicle weight of around 4000 lbs. My numbers may be off a bit but you get the idea.

4. Lower air dam extensions. Simple 3d-printed TPU parts that remain slightly flexible to keep air away from the front of the tire, which is a naturally turbulent spot. I've noticed new cars almost never leave the front of the tire exposed like our vehicles have, even trucks. Here is the passenger side extension.



5. Lower bumper plugs. There are large openings in the lower bumper that don't feed the power steering cooler or radiator. Blocking these off would lead to some nominal drag improvement.

For items 4 and 5 I have 3D-printed these parts out using TPU. I am actually in the process of printing the driver's side bumper plug. Here is the passenger side installed.


Once I get the design finalized a bit more I would be happy to share the .stl files for others to print their own if people are interested.

 

[masospaghetti]

Fairings for the rear wheel openings make a small aerodynamic improvement.

Can you elaborate? I assume you mean something like a small forward-facing splash shield (ala GMC Canyon shown here):

 

[Mektek]

No, a cover over the top half of the wheel well.
Like the Honda Insight:

https://upload.wikimedia.org/wikipedia/commons/1/1b/Honda_Insight_Back.JPG

 

[Mooseman]

I remember when they came out with the 1992 B Body Caprice. People complained so much that they went back to an open wheel well by 1994.

On a Prius or EV car, sure but not on an SUV or truck no matter the reason.

 

[TequilaWarrior]

 

[Mektek]

I wouldn't do it for a daily driver, rather a temporary installation for a long highway run. With five dollar a gallon gas in NY and the typical poor fuel economy of the platform - well - desperate times call for desperate measures

 

[TollKeeper]

Speaking of poor fuel economy.. Theres a person on the GMT facebook page that says she is getting 30mpg out of her LH6 because of the AFM/DOD. I call BS!

 

[Mooseman]

Same here. I saw no appreciable difference between DOD and no DOD. And that's with the Saab and the Caprice.

 

[masospaghetti]

There was also a guy on the Ford Explorer forums (I used to have one) that was claiming 35 MPG on a regular basis...

 

[flyboy2610]

There was also a guy on the Ford Explorer forums (I used to have one) that was claiming 35 MPG on a regular basis...

And I'm sure he drove downhill everywhere he went, too.

 

[paul2005tb]

When I bought my Trailblazer back in 01/2006 I was getting 21-22 mpgs. Of course I drive like an old lady. Now 16 years later with 3in of body lift and 3 in of suspension lift and 1.5in wheel spacers (many thanks to markmc!!) and 32.7in load range E offroad tires and Im getting 15mpg!!.. I think that is pretty good. Nevertheless it did seem odd that my OEM stock Trailblazer was getting at least 1 mpg less than the OEM Tahoe that my brother owned.

I wonder sometimes if I would get better mileage if my fuel injectors were tuned a shade leaner.

Nevertheless I do not regret my mods. I feel much better on the winter roads with the present set up.

 

[masospaghetti]

If you are really serious, you can save a few percent by filling your tank halfway instead of full. Half a tank of gasoline (11 gallons) weighs about 66 lbs, that's good for about 1.5% improvement. Of course, this may not be worth the trouble of filling up more often.

Also, if your gas pumps are not temperature compensated, filling up when the ground is the coldest (i.e. the morning) could get you another 1% or so. The fuel you get when it's relatively cool is more dense, so you get more fuel for the same price. Some pumps are temperature compensated so it depends where you live.

I had to reinstall my spare tire (and I fill up all of the way), but with fairly mixed driving I've averaged just about 20 mpg over my last three tanks.

 

[Mektek]

Weight only affects stop and go driving. For a long highway run making more frequent stops to refuel will cost you more than any potential savings.
For a tiny reduction in drag, remove the external antenna and install an internal window mounted antenna.

 

[l008com]

I've had my 5.3 TB for about 7 years now, wow. So I took the roof rack cross bars off early on for this very reason. It should be saving me some small amount of fuel every time I'm on the highway. It's impossible to know how much as I don't have a regular commute. I've always focused on weight for this type of thing. Anything you can do to lower the vehicle weight would help the city mpg a lot. And you'll be faster as a bonus.

Things I've always thought about (but never actually looked in to) would be a fiberglass replacement hood and front fenders. And maybe some aluminum or carbon fiber drive shafts to replace the ones there. I bet replacing the steel spare tire wheel with an aluminum one would save a few pounds.

If you really wanted to take it to the extreme, you could go without AC. That compressor is heavy and you could remove lines and heat exchangers too to save a lot of weight. You could go with manual seats. Not sure if you can mod the power seats and remove the motors and add levers, or if you'd need whole manual seats to fully swap in. For that matter, were any TB's made with *fully* manual seats?

You could replace the lead acid battery with a lithium ion battery. This doesn't work that great in cold climates but in warmer ones, this would work a lot.

That's about all I can think of. On my 5.3 short wheelbase, i typically get 13.5 to 14.5 mpg on a tank, which is mostly off-highway driving. If I happen to have a lot of highway driving, it can be anywhere from 15 mph all the way up to the best I ever got, 23 mpg on a tank. That was the day after I bought the truck. I drove to the gas station in bel air MD, filled up, and drive all the way home to Boston. I've never been remotely close to that since. I've hit 20 for a little while but it goes away fast once you get off the highway.

Oh I just read someone else mention the resonator removal. I did that too! Mainly because I have the V8 and the resonator just hides the V8 sound. Stock muffler but sans resonator makes the truck sound very respectable and not at all obnoxious. And saved me a few more pounds.

Also all this talk of transmission swap. if you're going to mess around with any kind of transmission swap, do the right thing and put a 6 speed manual in there. My truck would be AMAZING with stick. There's videos on youtube of someone with a very similar SWB 5.3 and a manual swap. It's amazing, I'm so jealous. Of course the engine light is on in every clip which in my state would mean no sticker and no drive. If this truck came with a manual option, and I were able to get a factory 5.3/manual 6 short wheel base back when i bought this in 2016, maaaaaaaannnnn i'd be pouring as much money as it wanted into this thing with the intention of driving it foreverrrrrrrr.

Also back on track here. I recently took all my caliper pins out, cleaned them, re-greased them and reinstalled them. The brake pedal felt better after that. If they're going on easier, they are almost surely coming off easier too. That probably saved me some gas.

 

[TollKeeper]

If you really wanted to take it to the extreme, you could go without AC. That compressor is heavy and you could remove lines and heat exchangers too to save a lot of weight. You could go with manual seats. Not sure if you can mod the power seats and remove the motors and add levers, or if you'd need whole manual seats to fully swap in. For that matter, were any TB's made with *fully* manual seats?

Actually yes, and you have to replace the entire seat frame..

As for the transmission.. I wonder if @limequat could delete some code in the PCM/TCM to get rid of the MIL light for a manual swap. Like borrowing some code from a Colorado with a stick or something.

 

[Mooseman]

As for the transmission.. I wonder if @limequat could delete some code in the PCM/TCM to get rid of the MIL light for a manual swap. Like borrowing some code from a Colorado with a stick or something.

This can be done very easily by any tuner. Any code can be turned off. There are even some parameters for manual trannys even if equipped with an auto.

 

[flyboy2610]

Be careful if you only fill your fuel tank halfway. I have read, and I'm only re-stating what I've read on the internet (so it must be true, right?), that the fuel pump depends on being submerged, or mostly so, in gasoline for cooling. Driving with excessively low fuel levels could lead to early failure of the fuel pump. Might not be worth 1.5% increase in fuel mileage.