GM Tire size calibration

[EfiBB69]

Just bought a regular cab/bed wt. I would like to install 20” wheels with 275/60s. Can anyone confirm that GM will support this on the wt trim and also what if any magic words to use for the service manager to understand what I’m asking?

[redwngr]

Did you use 'tire calculator' or similar to compare the revs per mile for the 20's vs what's on it?

 

You didn't indicate what tires it came with.

Assuming the standard tire for 2025 reg cab wt the standard tire -- QBN Tires, 255/70R17 all-season, blackwall  and compared to 275/60R20

(there are 4 others shown as available in the order guide)

 

'Tire calculator' shows only 1 mph difference at 70 mph 

 

 

Bigger diameter rim, but shorter sidewall ratio resulting in similar overall diameter. 

With deflected sidewalls give revs per mile 599 changed to 607 or 1.3% 

(607-599) /  599 x100% = 1.3% 

 

I'm thinking there is no need to be concerned about the small difference(s) between the two tires

 

[EfiBB69]

I got around 31” vs 33” diameter for the oem vs. the 275/60 20s that’s a 4mph  error at 65 with actual speed being 69ish.

 

Also, I’m sure the speedometer is not the only calibration change required. EPS and adas systems are probably affected as well. 

[silveradosid]

something wrong with that calculator, 255/70r17 is 30.9 inches

[Z45]

GM policy is to NOT provide a calibration (for changing tire size) UNLESS customer buys accessory GM wheel and tires.  The dealer can't do it without GM providing the calibration. 

 

GM offers tire calibrations for tires that have been sized, tested and designed for the vehicle in question and its applications. Other tires that do not meet these qualifications will not be supported and tire calibrations will not be available.

Edited April 24 by Z45

[TA^Guy]

On 4/23/2025 at 7:46 AM, silveradosid said:

something wrong with that calculator, 255/70r17 is 30.9 inches

Agreed.  a 275/60R20 will be about 2" taller. 

[PhilB]

On 4/23/2025 at 6:46 AM, silveradosid said:

something wrong with that calculator, 255/70r17 is 30.9 inches

I think the "COMPARE" button was not pressed when the 20" tire was entered.  If you look at the right side of the picture where the tires are compared, it is comparing an 18" tire to a 17" tire.

[EfiBB69]

So now that we have resolved the issue with the tire size calculator. Does someone have actual experience with asking and being told no.

[CKNSLS]

I asked before I decided against changing and the answer from the dealer was "GM won't let us do this".

 

He did add- some after market tune shops could probably do it.