Wrong Factory Tire Size?

[AdubZ28]

I just downloaded the tune on my 08 Silverado and the factory tire diameter is set at 30.57 (245/70 17), but my truck came stock with 31.6" tires (265/70 17s). Is this a common occurance?

[Russ K]

That is normal, the tire size used in the tune file is always less than the advertised or measured tire dia.

Russ Kemp

[Super73]

Russ, why is this? In the file you sent back to me, I think you put like a 25.3" tire vs the 26.1 I use. My speedo @ 65 is fast by 2mph right now using 26.0 as my tire size in the HPT file. I would assume using the 25.3 tire it would make it read even higher than 2mph. I was planing on putting in a 26.2 in the file to see where that puts me. I'll be driving by one of those radar deals this evening.

[xsports33]

mine does the same thing. My tires are rated at 35" tires but i have to program them as 33.5" tires for my speedometer to read correctly. I used GPS to ensure that my speedo was spot on.

[454SuperSport]

measure the tire diam. with it mounted on the truck. gm accounts for the tire "squish" when mounted on the vehicle, thats why it doesn't match the advertised height.

[Programmer]

I just downloaded the tune on my 08 Silverado and the factory tire diameter is set at 30.57 (245/70 17), but my truck came stock with 31.6" tires (265/70 17s). Is this a common occurance?

By the math, a 245/70-17 is (245mm less 70% section width) X 2 + 17 = 30.5"

265/70-17's are 31.6" as you state--but are only 3% different in height and rotations per mile.

Rotations per mile for a factory tire should be -- 661
Rotations per mile 31.6" tire is -- 638.7

Revolutions per mile X Diff Ratio (I'll have to guess here) X 40pulse reluctor = Pulse per mile...
so assuming 661 x 3.73 x reluctor pulse (40-44) = 98621 ppm
assuming 638 x 3.73 x reluctor pulse = 95294 ppm

No matter how you work it, it's always 3% -- which fits right in with their error tolerance.

I hope that explains it a bit further....

[straped for cash]

Its too late for my brain to do math, but the Tire Rack says my tires are 30.1" and my current tire size is set at 29.23, that should be under 3%. Is there a formula or estimation for said "squish" or should we just use a yard stick for the diameter? Isn't it true that tires grow as speed increases, so it would be off anyway?

[Programmer]

Yes, we've also seen a lot of incorrect info from tire manufacturers.

One thing you can do is mark your tire--mark the floor...roll it one complete rotation. Mark the floor again--measure the distance between the two spots...and you'll nail it every time.

[xsports33]

another way would be to use a GPS to check your speed then play with your tire diameter until your speedometer matches your GPS. Then you'll know your tire size.

[Nimrod_R]

In newer cars (of the last few years). If the factory computer has all these certain settings and you change your tires (bigger or smaller), wont the car still read the same speed cause the speedo goes off of the computer settings. The computer doesnt know you changed tires, so wouldnt the speedo still read correct.

[PitchblackC3]

Old thread, but I was wondering the same thing.
On my 2001 GMC the stock tire size is 265/70/r17, I have replaced them with 305/40/r22 which according to calculations are the exact same height. I know I won't have as much flex in the sidewall, so I guess I will check with GPS and adjust accordingly.

[Dafuco0901]

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[PitchblackC3]

Calculating for tire squish doesn't make sense. No matter how much you squish a tire it still has the same amount of surface to roll before it does a complete revolution.