Odometer off - Re calibration

[steve rooney]

Have you ever check your odometer to see if it's correct? I did and it's almost exactly 1/10th off per mile. I did the math for about 170K miles and that equals 17000 miles above actual. Anyone ever have their odometer re-calibrated? I have the same tires that came from the factory so no changes there. Very curious about re-calibration of odometer.

UPDATE - I figured it out. I had to buy new tires and the odometer is calibrated to the exact size. Tire wear (my were worn but not to a level they were unsafe, just needed new) on your tires causes the odometer to be incorrect (kind of a, duh, comment) but who knew a 1/10th of a mile? Just seems like a lot over the life of a car.

 

[swathdiver]

Mine is spot on with fresh tires. Now it's 1 mph slower than what the speedo says as the tires are not the same diameter anymore as they have worn down some. I thought it would not matter on GMT900 because of wheel speed sensors but apparently still does.

 

[Tonyrodz]

Crazy that worn tires makes a difference in calibration.

 

[swathdiver]

Crazy that worn tires makes a difference in calibration.

Consider this:

Mathematically, a 265-70-17 tire is 31.61" tall with a circumference of 99.29" and 638 revs per mile. Doing 70 MPH with a 4-Speed in 4th shows the engine turning about 1775 rpms.

Using the KO2 tire as an example, it's tread depth is 15/32. So that applies to the above.

Now let's say the tread depth is 10/32, the tire is now 31.29" tall, 98.31 inches around and does 644 revs per mile. It also loses 1 mph in 4th with the engine turning 1775 rpms.

Not all tires of the same size are the same size either. And this can get more out of whack when different width wheels are used too.

 

[adriver]

UPDATE - I figured it out. I had to buy new tires and the odometer is calibrated to the exact size. Tire wear (my were worn but not to a level they were unsafe, just needed new) on your tires causes the odometer to be incorrect (kind of a, duh, comment) but who knew a 1/10th of a mile? Just seems like a lot over the life of a car.

There is absolutely no way that tread wear only is causing a 10% difference in your speedometer.

 

[steve rooney]

There is absolutely no way that tread wear only is causing a 10% difference in your speedometer.

Open to any other ideas/answers why my odo would be off 1/10th on my old tires. In fact, checked my odo RIGHT BEFORE I went in for new tires wondering if tire wear might have been the issue. On my ride, immediately checked the odo and BAM, I'm back at accurate.

 

[adriver]

Consider this:

Mathematically, a 265-70-17 tire is 31.61" tall with a circumference of 99.29" and 638 revs per mile. Doing 70 MPH with a 4-Speed in 4th shows the engine turning about 1775 rpms.

Using the KO2 tire as an example, it's tread depth is 15/32. So that applies to the above.

Now let's say the tread depth is 10/32, the tire is now 31.29" tall, 98.31 inches around and does 644 revs per mile. It also loses 1 mph in 4th with the engine turning 1775 rpms.

Not all tires of the same size are the same size either. And this can get more out of whack when different width wheels are used too.

Yeah and OP originally said it is 10% off, 1/10th off per mile.

A 265-70-17 tire has a tire circumference of 99.27" and a height of 31.61"
To be 10% off the tire circumference would need to be 99.27 - (99.27/10) = 99.27-9.927 = 89.343"

89.343/ (2 x Pi) is the formula for circumference. = 14.2193801 (radius)
14.2193801 x 2 = 28.4387602 diameter / tire height
31.61 - 28.4387602 = 3.1712398"
3.1712398" = 80.5494909mm

For your speedometer/odometer to be off by 10% because of tread depth you would have needed to have (80.549mm / 2 =) 40 mm of tread depth... or (3.171" / 2 =) 1.585".

 

[adriver]

Open to any other ideas/answers why my odo would be off 1/10th on my old tires. In fact, checked my odo RIGHT BEFORE I went in for new tires wondering if tire wear might have been the issue. On my ride, immediately checked the odo and BAM, I'm back at accurate.

Are you sure you mean 10%?
What is your RPO codes that start with "Q" and "G"?
Are you sure your old tires and new tires are the same size?

The Q code is your factory tire size that your vehicle was calibrated for.
The G code is your rear end gearing size. It might be different, but I think it should be GU

 

[adriver]

OP, you didn't even say if you were reading faster or slower than actual speed.

 

[steve rooney]

Are you sure you mean 10%?
What is your RPO codes that start with "Q" and "G"?
Are you sure your old tires and new tires are the same size?

The Q code is your factory tire size that your vehicle was calibrated for.
The G code is your rear end gearing size. It might be different, but I think it should be GU

Everybody's getting all mathy on me. Not a math guy but can read an odo and it was 1/10 of a mile off. But... I do appreciate the input and ideas. Still kind of an interesting topic. I thought the deal was kind of interesting. What really got my attention, when I called the dealer and asked about re-calibrating. The struggled to talk with me and directed me to GM. GM got super sketchy on me like a touched a very sensitive topic. I think I did strike a raw nerve with GM since odo's are to be accurate buy law.

RE: my RPO codes - that was the first thing I checked before calling the dealer. Everything matches up and tires that were on the vehicle vs. now are the same size, different brand.