New larger tires

Dieseldog

Registered Member
Original poster
Jul 22, 2020
25
New Mexico
I finally got my wheel spacers and tires mounted on my rig!! I think it looks great now! I got 1 1/2" wheel spacers and my tires are 245 75 17. I drove it through some bumpy roads to check if I would get any rubbing anywhere but it all seemed fine even through a left and right u-turn. No rubbing. I think the next thing I'm going to do is an 1 1/2" front leveling kit.
 

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flyboy2610

Active Member
Aug 24, 2021
301
Lincoln, Ne.
I have the same setup on my '04 TB EXT. I really like the way it rides!
As far as leveling, I would steer clear of the spacers that simply bolt to top of the struts. I currently have the ReadyLift kit on mine, and will be replacing it with the BDS 2" strut mount replacement style lift. I have come to find out that the BDS style is much better for the suspension. Has to do with spring preload is my understanding.
They make a kit which includes the components for the front and rear. I bought the one only for the front since I already have the spacers for the rear.
My wife doesn't like to ride in my TB, she says it's too high! It's only 3" higher than stock... for now! 😁
 
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mrrsm

Lifetime VIP Donor
Oct 22, 2015
6,275
Tampa Bay Area
Hmmm...

TIRESIZESPEEDCOMPARISON.jpg
HEISENBERGUNCERTAINTY.jpg
 
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Mooseman

Master Blaster
Moderator
Dec 4, 2011
23,038
Ottawa, ON
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mrrsm

Lifetime VIP Donor
Oct 22, 2015
6,275
Tampa Bay Area
The comment I made here casts nothing pejorative towards the OP. Of course he realizes the obvious. But THIS issue can become a Pebble in Everyone's Shoe because of it's "uncertainty". And it's THAT "Heisenberg Issue" that would just Drive Me NUTS.

When driving through our adjacent 'Mom-N-PoP' Communities down here, they have Municipal Police who'll pull people over for driving as little as 5 MPH Over the Speed Limit.

And so this 'little' problem can cascade and become very annoying because If they FEEL like it... The local cops can write citations that can require 'The Elderly' to have to show up at the FLDHSMV and take a Physical Driving Test just on their Say-So.

So its THAT issue ...and the fact that there isn't some simple way to easily re-factor in these Tire OD Changes *** directly into the Speedometer ***, without requiring either a Tech 2 or having to use HP-Tuners to make these subtle adjustments.

That's All...
 
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mrrsm

Lifetime VIP Donor
Oct 22, 2015
6,275
Tampa Bay Area
Hmmm... I've been thinking about this issue a bit more...

Since the Speed and Tach are controlled by inputs to the PCM and BCM and uses Pulse Width Modulation... (PWM) at the Instrument Panel Cluster (IPC) to register on the Gauges as "Faux Mechanical Analog" read outs...

... I'd 'Bet a Brace of Gutenberg Bibles' that @TJBaker57 could probably Cook us up some Brilliant Cell Phone Torque APP Speedo Tweak in that Digital Alchemy Lab inside of his Amazing Brain that can display these Adjusted Speeds ...with Absolute Precision.

Then all he'd need to do is plug in those" Stock and Modified Tire Parameters" I gleaned in that image from the On-Line Tire Comparison Calculator attached in @Dieseldog 's #1 Post to figure this 'Whole Tire Thing' out. :>)

Tire Pressures would be critical in the long run of course, especially under Hard Acceleration as their Outside Diameters increase from Transitional Velocity and Centrifugal Force. Note THIS Dramatic Example happens at 2:42 into THIS Cool Video:

 
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azswiss

Active Member
May 23, 2021
487
Tempe, AZ
Great idea @mrrsm! Working from the Transmission Output Speed PID (221942) and factoring in Wheel Diameter & Axle Ratio data I was able to create a custom Corrected Speed PID. Compared it against GPS and got good correlation.

The PID equation details are listed below:
OBD2 PID: 221942
Long Name: Corrected Speed
Short Name: Corr_Spd
Minimum: 0.0
Maximum: 125.0
Scale Factor: x1
Unit type: MPH
Equation: ((A<8)+B)*.000371*WD/AXLE
- WD = Effective Wheel Diameter in Inches (need good link for this data)
- AXLE = Rear Axle Ratio
OBD Header: Auto

Edit: Fine tuning of this equation likely required based on comparison of calculated corrected speed vs. actual measured via GPS. Set cruise control, note Corr_Spd & GPS speed, then multiply the .000371 constant value by a correction factor calculated as the ratio of Observed GPS Speed to Calculated Corr_Spd.

Edit #2: Fixed error in the equation (forgot to remove my wheel diameter & axle ratio)
 
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Dieseldog

Registered Member
Original poster
Jul 22, 2020
25
New Mexico
It's funny that you brought the speedo issue up cause the first thing I did was drive by the little radar box that is setup in my town!! It appeared to be off by 3 mph. We are going camping this weekend and I wanted to get the tires on before we went cause the tires that were on it were still the ones from when I originally got it and they had to be at least 8 years old if not older!!!
 
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Joe_67

Registered Member
Aug 9, 2022
40
Central Virginia
I have an '02 Envoy SLT running on 235/75-17s, so a hair smaller. Using any kind of GPS speedo type of app (including on Waze or whatever), my speedo runs about 3mph under. Keep in mind too that in the long term it slows the ODO down a bit (unless there's something I don't know about the ODOs in these things.
 

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