Odometer Adjustment for Slightly Larger Tires

DenaliHD66

Original poster
Member
Dec 4, 2011
597
So I always knew that larger/smaller tires would affect your speedometer reading, but I was recently introduced to the idea that it also affects your overall mileage.

My OEM tire size is 245/65/17, and at 40k miles I switched to 255/65/17. Using an online tire calculator, it says that this new tire size is 1.7% slow, so when my speedometer says 60 mph, my actual speed is 61 mph.

So basically, for every 100 miles, there are 1.7 miles unaccounted for on my odometer. 60k miles driven on with the larger size tires, means that there are 600 [100's] in that space. So, 600x1.7 = 1,020 miles that are not accounted for on my odometer. Does this sound right to anyone else? I understand its not going to be exact but I'm just curious if it is quite possible that my odometer is off by that much.

Where exactly is the sensor(s) in our trucks anyway and how do they work for the speed/mileage? Wouldn't it be possible to design an internal one that goes by the transmission turning or something, so it would be devoid of the tire size factor?
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
The driveshaft rotations are sensed by a sensor on the tailpiece of the transmission or the transfer case. The pulses get sent to the PCM for various purposes, and the computer has a table built into it that converts pulses into miles (conversion factor is called pulses per mile). Thus the PCM must know how large your tires are and the gear ratio of the differential. But GM made it difficult to change that conversion factor, but it's been done by various folks who tune the PCM for all the other benefits you get from a tune. The only "authorized" PPM numbers are the ones that GM shipped as normal options, and it's been difficult to succeed getting the right MPH displayed for upsized tires.

Your odometer is indeed reading low by the same factor, but don't feel guilty when selling the vehicle, because the drivetrain wear is proportional to engine revs, not the mileage you put going down the road. Only tire wear is proportional to mileage, and it can be argues that revolutions is a better measure of that as well.
 

DenaliHD66

Original poster
Member
Dec 4, 2011
597
I don't and won't feel guilty at all :tongue:. I just wanted to clarify whether or not what I thought was going on was actually accurate. And I figured it would be dumb to have any mileage sensors on any wheel, given that not every wheel will always turn, but at least 1 will be turning when the driveshaft turns.
 
Jul 23, 2012
20
If you REALLY feel the need to adjust the tire size in the PCM, find either the overall height or revolutions per mile for the tires you are using now. Then take those figures and check the tire size charts to determine what sizes are the closest to your stats and make a list of those tire sizes. Then get someone with a Tech II (it can be the dealership, but many others have them) and reset the tire size in the PCM using the closest you can find to the size you are using in the list that is displayed in the Tech II. I have done it on several vehicles and if close, the difference will often times be 0.5-1.0% off, which is about as close as you can get.
 

Irishboy02

Member
Apr 1, 2012
222
I traded my tb with 72k on the clock. Had 30k on the motor, 7k on the trans and about 85 on the chassis haha. Im scared to even drive past that dealership again. Just need to be mindful of the whole speed thing. Your not that far off so its not that bad. I was on 33s so there was a pretty big difference
 

Sparky

Member
Dec 4, 2011
12,919
Silver Trooper said:
If you REALLY feel the need to adjust the tire size in the PCM, find either the overall height or revolutions per mile for the tires you are using now. Then take those figures and check the tire size charts to determine what sizes are the closest to your stats and make a list of those tire sizes. Then get someone with a Tech II (it can be the dealership, but many others have them) and reset the tire size in the PCM using the closest you can find to the size you are using in the list that is displayed in the Tech II. I have done it on several vehicles and if close, the difference will often times be 0.5-1.0% off, which is about as close as you can get.

The issue is the 02-05 Trailblazers are really finicky about such adjustments. People have tried and it causes all sorts of strange things to happen. PCM4Less won't even attempt a tire size adjustment on those years because of how screwy they are.

Kinda sucks because I want to go from the stock 28.5" to 32" tires and that will throw things pretty far off (about 11%).
 
Jul 23, 2012
20
Sparky said:
The issue is the 02-05 Trailblazers are really finicky about such adjustments. People have tried and it causes all sorts of strange things to happen. PCM4Less won't even attempt a tire size adjustment on those years because of how screwy they are.

Kinda sucks because I want to go from the stock 28.5" to 32" tires and that will throw things pretty far off (about 11%).

That is very strange as I have had at least four or five vehicles (three Silverados and two Suburbans if I remember correctly), and I never had a problem with any of them. I think I even did two of them myself with the Tech II. What do you mean by "finicky"? What types of problems have resulted from trying to reset the tire size? I would think that as far off as yours is you would have to be able to get at least somewhat closer to an accurate reading.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Rumor from tuners is that there are THREE different tables where the tire size can be adjusted. Not every tuning software can get at them all. Even PCMforless is unable to get it right on my 2004, and there may be others, IIRC.
 

Sparky

Member
Dec 4, 2011
12,919
Silver Trooper said:
That is very strange as I have had at least four or five vehicles (three Silverados and two Suburbans if I remember correctly), and I never had a problem with any of them. I think I even did two of them myself with the Tech II. What do you mean by "finicky"? What types of problems have resulted from trying to reset the tire size? I would think that as far off as yours is you would have to be able to get at least somewhat closer to an accurate reading.

It has to do with the PCM on the i6 for those years. 2006+ has a more capable PCM.

One issue I'm aware of was the speedometer going absolutely crazy after an attempted change, another was the speedo didn't change at all but something else no longer worked right (shift points were off or cruise refused to work, something). Those are the two that I remember.
 

jbones

Member
Dec 5, 2011
658
I also put 255/65/17's on.

Without any tricorder, calculator, pencil and paper I’ve driving several different mile markers and Speedo checks on the highway, and have always been damn near on top at the end. Works for me! My wife is always yelling you have to drive 69 now, I tell her wait til we get home, I always get smackced.
 

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