Musings about the earlier GMT360s, tire sizes, gear ratios, and the PCM

Sparky

Original poster
Member
Dec 4, 2011
12,927
From what I understand the 2002 - 2005 GMT360 PCM cannot be tuned for tire size. I know some people had played with doing it but the consensus was it isn't possible. Why is this? What happens when you try?

The most common 16" tire size to upgrade to when lifting is 265/75/16, which is a 31.6" tall tire, vs the stock 28.5" tire. Now here's where I started musing, plus I'm a little tired, so be warned :tongue:

Stock tire circumference is 89.7 inches. The upgraded tire is 99.4 inches. So we're looking at about 10.8% increase, which means if nothing else is changed the speedometer will be reading 10.8% too slow (reads 60mph, actually going 67mph). This also means the odometer isn't putting on the miles fast enough.

The most common gear ratio is 3.42. A 3.73 gear ratio I've been told seems to be a better fit for these trucks, particularly when lifted with the bigger tires. That ratio is about 9% lower. If you change the gear ratio without changing anything else, the speedometer reads 9% too fast (reads 60mph, actually going 54.6mph). Odometer is now racking up miles faster than reality.

So... What if someone crazy (like me) was to put the taller tires on and change the differentials? They should counteract each other, meaning the end result would be the speedometer and odometer would only be 1.8% too slow (reads 60mph, actually going 61.1mph), plus I'd have the better gearing.

I already replaced my rear axle once when my old one was worn out, and I know the front is a huge pain to swap out, but it was just some ideas rolling around my head. I guess if I was to have front diff issues that required me to take stuff apart up front it would make more sense to do that then.

Anyway, any thoughts on this or am I totally whacked out and need to get some sleep :rotfl:
 

McGMT

Member
Jun 17, 2012
621
Sparky said:
plus I'd have the better gearing.

Anyway, any thoughts on this or am I totally whacked out and need to get some sleep :rotfl:

Well, no you wouldn't because you would only be changing the gearing by the 1.something percent in the end... So your not changing anything other than the look of the tires... And, Goodnight buddy.. lol
 

Sparky

Original poster
Member
Dec 4, 2011
12,927
I meant I'd have the better gearing as in better suited for the bigger tires vs keeping the 3.42, along with getting the speedometer and odometer more accurate again.
 

McGMT

Member
Jun 17, 2012
621
Sparky said:
I meant I'd have the better gearing as in better suited for the bigger tires vs keeping the 3.42, along with getting the speedometer and odometer more accurate again.

haha yea, you would, for those tires.. DER, Im tired too sparky come on! :crazy::crazy:
 

RayVoy

Member
Nov 20, 2011
939
Sparky, I didn't check your math, but if the end result is the same (or close to same) overall gearing ratio, then the performance with larger tires would be the same as the OEM performance.

You have gained larger tires without losing any performance.
 

Sparky

Original poster
Member
Dec 4, 2011
12,927
So then the question is whether it is worth tearing out the rear axle again and yanking the front diff :crazy:
 

strat81

Member
Dec 29, 2011
399
Sparky said:
From what I understand the 2002 - 2005 GMT360 PCM cannot be tuned for tire size. I know some people had played with doing it but the consensus was it isn't possible. Why is this? What happens when you try?

Really? The 2004 Buick Rainier was offered with three different wheel sizes. It seems odd that there isn't a changeable setting somewhere in the PCM for tire size.
 

RayVoy

Member
Nov 20, 2011
939
Sparky said:
So then the question is whether it is worth tearing out the rear axle again and yanking the front diff :crazy:
To gain a little clearance.

strat81 said:
Really? The 2004 Buick Rainier was offered with three different wheel sizes. It seems odd that there isn't a changeable setting somewhere in the PCM for tire size.
I'm not saying the Buick is like this, but my Avalanche has 3 different tire/wheel sizes (depending upon model and options), all three have the same overall diameter and circumference. Therefore, no need to program the PCM.
 

HARDTRAILZ

Moderator
Nov 18, 2011
49,665
There are gear ratio calculators out there. I have a better effective ratio with my 35s and 4.10s than I did with my 3.73 and 33s. I believe my effective is about the same as 3.55 w stock tires. But the gearing in out tranny's and the overdrive make most calculators just a bit off and real world...I am fairly happy with 35s and 4.10s. The 33s and 4.10s were a bit better, but with the shift points and such in my tune not being all worked out and in sync...it is hard to know. I am planning to send my PCM back in for a final tune and some codes deleted and get the tire size matched with the speedo and shift points so I will let you know how upsizing tires and compensation via gearing really works soon.
 

strat81

Member
Dec 29, 2011
399
RayVoy said:
To gain a little clearance.

I'm not saying the Buick is like this, but my Avalanche has 3 different tire/wheel sizes (depending upon model and options), all three have the same overall diameter and circumference. Therefore, no need to program the PCM.

It's possible, based on what tolerance GM allows for the variation.

The base size is 245/65-17, 29.5" diameter.

The first optional size is 255/60-17, 29.0" diameter, speedometer reads 1.7& too fast.

The second option size is 255/55-18, 29.0" diameter, speedometer reads 1.7& too fast.

Seems like something that would be easy to adjust, at least with the proper tool. A $50 parts store unit wouldn't do it, but I'd be surprised if a Tech2 couldn't do it. Of course, whether or not the dealer will change that setting or would know how to is a different issue.

Maybe HPTuners or EFILive could get it done?
 

HARDTRAILZ

Moderator
Nov 18, 2011
49,665
strat81 said:
Seems like something that would be easy to adjust, at least with the proper tool. A $50 parts store unit wouldn't do it, but I'd be surprised if a Tech2 couldn't do it. Of course, whether or not the dealer will change that setting or would know how to is a different issue.

Maybe HPTuners or EFILive could get it done?

The point of this is that the Tech2 can't do it and none of the tuners have been able to. So he wants to change gear ratios with tire size to keep the speedo ok.
 

Sparky

Original poster
Member
Dec 4, 2011
12,927
Yeah I'm not sure why the PCM can't allow for tire changes either. My 99 Camaro can and it's 3 years older.
 

ScarabEpic22

Member
Nov 20, 2011
728
strat81 said:
Really? The 2004 Buick Rainier was offered with three different wheel sizes. It seems odd that there isn't a changeable setting somewhere in the PCM for tire size.

strat81 said:
It's possible, based on what tolerance GM allows for the variation.

The base size is 245/65-17, 29.5" diameter.

The first optional size is 255/60-17, 29.0" diameter, speedometer reads 1.7& too fast.

The second option size is 255/55-18, 29.0" diameter, speedometer reads 1.7& too fast.

Seems like something that would be easy to adjust, at least with the proper tool. A $50 parts store unit wouldn't do it, but I'd be surprised if a Tech2 couldn't do it. Of course, whether or not the dealer will change that setting or would know how to is a different issue.

Maybe HPTuners or EFILive could get it done?

EFILive and HPTuners can do some of the 02-05s, problem is sometime in 03/04 GM changed how the ABS module and PCM report the speed to one another (IIRC). So you can change it in the PCM and either get 0 change or weird things like the speedo will read 60mph and the databus will read 40mph when you're actually going 30mph. And then sometimes it will be 40mph and match the databus, but the speedo still reads incorrectly. Believe me, I spent a few days working on it then gave up as only one of my customers runs bigger tires (and Craig said screw it and uses his DashHawk anyway).

HARDTRAILZ said:
The point of this is that the Tech2 can't do it and none of the tuners have been able to. So he wants to change gear ratios with tire size to keep the speedo ok.

Actually the Tech II could, at least at one point. I dont know if GM updated their software/hardware and it cant now, but I know when the TBs first came out dealers could adjust the tire size.

Sparky said:
Yeah I'm not sure why the PCM can't allow for tire changes either. My 99 Camaro can and it's 3 years older.

Difference in PCMs, the 99-02 LS1 512kb PCM has proven itself to be THE most versatile PCM GM (and arguably every other manufacture) has ever made. You can run almost any V8 from LS1, LS6, LS2 (24x only), LT1 and LT4 (conversions), 96-00 Vortec 305, 350, 454s. Throw in the 03-08 LS1 1MB PCM and you can control ETC as well. There are guys using them to run I4s from Isuzu and I think Porsche as well plus GM I4s. Seriously, if they could run a V6 and/or I5/I6 and run 58x engines natively, basically any GM engine from 96-today could be controlled by it. Not in a factory chassis in some cases, but in swaps no problem. SUVs, trucks, Vettes, Camaros, etc all used it. Any vehicle GM made from 00-04 used the LS1 PCM, then the Vette started with the Gen IV ECM/TCM setup in 05 but everything else had it until 06 (at least).

And then there's our lovely P10, made ONLY for the GMT360/370/305 trucks and ONLY for the I6 and ONLY from 02-05. Plus I know from talking with other tuners who were looking at the actual code in the PCM that it was passed off to no less than 3 SEPARATE development teams which means tracing any tables/values/logic takes forever and sometimes doesnt work.

So when EFILive/HPTuners/TunerCats looks at how much time/resources to devote to a project, not surprising that the LS1s get WAY more time. Ive been a huge thorn in EFILive's side advocating for more tables, got 2 relatively major updates from it over 5 years. I think the LS1 cals have got close to 94 updates (both major and mostly minor) since EFILive released support for them in 05. LS1s have a cal version of 7.94 while P10s are 7.10 (should be 7.11 though).

Better than nothing, but when I went to SEMA they made me promise NOT to talk about the P10 when I met them! Hahaha, I did but that lasted about 10 min. Fortunately, THEY brought it up so I didnt feel bad talking their ears off. :biggrin:

Hope that clears it up a little.
 

Sparky

Original poster
Member
Dec 4, 2011
12,927
Well actually my Camaro isn't the LS1, just a (cammed) v6 so the PCM there is different. Still supports tire changes though, but maybe because that 3800 PCM was used in a few other vehicles that engine was also used in (Buicks, Pontiacs, both FWD and RWD platforms, etc).

I suppose I would do the lift/tires first and see how it does. If the speedo and/or performance really bugs me a lot then look into the diff changes, I guess. The rear doesn't bug me aside from cost (though I could resell mine easy enough), but the front, well, :crazy: This is all looking into the future however, I doubt much of this would happen this year at this point. There is still a slight chance the lift might depending on how my tires look later this year before winter gets here, but as of right now they still look decent even with 42k miles on them. I need to measure the tread depth to know for sure.
 

Sparky

Original poster
Member
Dec 4, 2011
12,927
The V8s use the same PCM as all the other V8 trucks IIRC, so no they shouldn't have any annoying limitations like that.
 

Pittdawg

Member
Dec 5, 2011
538
Sparky said:
Well actually my Camaro isn't the LS1, just a (cammed) v6 so the PCM there is different. Still supports tire changes though, but maybe because that 3800 PCM was used in a few other vehicles that engine was also used in (Buicks, Pontiacs, both FWD and RWD platforms, etc).

I suppose I would do the lift/tires first and see how it does. If the speedo and/or performance really bugs me a lot then look into the diff changes, I guess. The rear doesn't bug me aside from cost (though I could resell mine easy enough), but the front, well, :crazy: This is all looking into the future however, I doubt much of this would happen this year at this point. There is still a slight chance the lift might depending on how my tires look later this year before winter gets here, but as of right now they still look decent even with 42k miles on them. I need to measure the tread depth to know for sure.

Get the gears!!!
 

Sparky

Original poster
Member
Dec 4, 2011
12,927
I just looked up the process in my Haynes for removing the front diff. Almost total disassembly of the front end and yanking the oil pan :crazy: Time and lots of $$$$ involved in doing a 3.73 gear upgrade. Probably an easy $1000 by the time I get yet another new rear axle, front diff, and all the other misc stuff I'd need. Yeah that won't happen this year! If I did the lift this year any gear change would have to wait until next, at least. Granted I could resell my rear axle and probably my front differential also to help offset the cost.
 

ScarabEpic22

Member
Nov 20, 2011
728
Sparky said:
Well actually my Camaro isn't the LS1, just a (cammed) v6 so the PCM there is different. Still supports tire changes though, but maybe because that 3800 PCM was used in a few other vehicles that engine was also used in (Buicks, Pontiacs, both FWD and RWD platforms, etc).

I suppose I would do the lift/tires first and see how it does. If the speedo and/or performance really bugs me a lot then look into the diff changes, I guess. The rear doesn't bug me aside from cost (though I could resell mine easy enough), but the front, well, :crazy: This is all looking into the future however, I doubt much of this would happen this year at this point. There is still a slight chance the lift might depending on how my tires look later this year before winter gets here, but as of right now they still look decent even with 42k miles on them. I need to measure the tread depth to know for sure.

So HPT (and TunerCats?) has parameters for it, EFILive doesnt support the 3800 PCMs unfortunately. But, the main concept of my post remains, there were still a bunch of V6s that all SHARED the same PCM.

strat81 said:
Scarab, thank you for the detailed post.

Do the V8 GMT360s have the problematic PCM the I6s have?

Sparky said:
The V8s use the same PCM as all the other V8 trucks IIRC, so no they shouldn't have any annoying limitations like that.

No problem, and the V8s do not:

03-04 LWB 5.3: LS1 PCM
05-06 5.3, 06 6.0: E40/T42 combo
07-08 5.3, 07-09 6.0: E67/T42 combo.

And they dont have the annoying limitations, all the V8s have controllers fully supported by EFILive, HPTuners, and TunerCats/JET DST.

Sparky said:
I just looked up the process in my Haynes for removing the front diff. Almost total disassembly of the front end and yanking the oil pan :crazy: Time and lots of $$$$ involved in doing a 3.73 gear upgrade. Probably an easy $1000 by the time I get yet another new rear axle, front diff, and all the other misc stuff I'd need. Yeah that won't happen this year! If I did the lift this year any gear change would have to wait until next, at least. Granted I could resell my rear axle and probably my front differential also to help offset the cost.

Yep, its pricey. I want to swap the 4.10s in my SS for 3.42s in preparation for my 6 speed upgrade, but the front gears alone are like $400. Then have to pay someone to install them (I can pull the diff, but not going to try and set the gear lash/etc). Then on to the rear, fortunately those are a lot easier. I think I guesstimated around $1200 for new gears and install fr/rr. :lipsrsealed:
 

Sparky

Original poster
Member
Dec 4, 2011
12,927
Is the front diff on the SS the same as the others? If so... BUY MY FRONT DIFF :rotfl: never mind that it has 161k miles and counting :tongue:
 

Pittdawg

Member
Dec 5, 2011
538
ScarabEpic22 said:
Yep, its pricey. I want to swap the 4.10s in my SS for 3.42s in preparation for my 6 speed upgrade, but the front gears alone are like $400. Then have to pay someone to install them (I can pull the diff, but not going to try and set the gear lash/etc). Then on to the rear, fortunately those are a lot easier. I think I guesstimated around $1200 for new gears and install fr/rr. :lipsrsealed:

I still have my 3.42's laying around with much less mileage than Sparky's if you and Sparky just want to swap out entire front and rear differentials...
 

ScarabEpic22

Member
Nov 20, 2011
728
Sparky said:
Is the front diff on the SS the same as the others? If so... BUY MY FRONT DIFF :rotfl: never mind that it has 161k miles and counting :tongue:

Yep, all the front diffs are the same (IIRC), its just the engagement fork for 4WD vs AWD that are different (roadie, can you verify this?).

Pittdawg said:
I still have my 3.42's laying around with much less mileage than Sparky's if you and Sparky just want to swap out entire front and rear differentials...

Hmmm, thats very tempting and thanks for the offer! Im not going to be doing this for a while, maybe next year if everything works perfectly. More of a long-term mod plan, not sure what my schedule is going to be as I just started a new job and I dont know if I can afford to have my SS down for a while while I do the swap.
 

DenaliHD66

Member
Dec 4, 2011
597
So I have 3.42 gears, and if I swapped them for 3.73s or 4.10s my speedo would be too fast and accumulating mileage at a faster rate?

Someone link me to the thread that talks about the options for upgrading gears please :biggrin:
 

Sparky

Original poster
Member
Dec 4, 2011
12,927
Yes. If you swap for a numerically higher gear number without changing anything else, the speedometer (and odometer) will be read faster than actual. That's why I was thinking I could offset the effect of larger tires on the speedometer by swapping gears. I'm not going to do that however. Too much work to get to that front differential and my truck is rusty enough I don't want to sink too much into it that I can't just relatively easily move to the next GMT360 after this one bites the dust.
 

Forum Statistics

Threads
23,428
Posts
639,193
Members
18,646
Latest member
rjs1026

Members Online