Factory Speedometer Calibration

mistermcgoo · October 31, 2019

So of a random post, but I’ve seen this asked in other threads in the past..

I just picked up a 2018 z71 and swapped my replica GMC honeycomb 20” wheels I had on my 2016 LT to this truck. This 2018 came with 20” wheels from the factory, my 2016 came with 17’s originally.

I noticed that my speedo was off by about 3-4 mph at around 70 mph after adding the 20’s. This 2018 is either dead on accurate or off by maybe 1/2 mph at the most..

I know a lot of people have wondered whether or not our trucks come from the factory with a general calibration or a “wheel specific” calibration based on whatever wheel size that particular trim came with originally.

Based on what I’m seeing, it seems to be specific?

Lsgun1 · October 31, 2019

What size tires?

mistermcgoo · October 31, 2019

I have 275/60r20's on the wheels I swapped over, the original tires on the factory 20's were 275/55r20 I believe..

ray2005 · October 31, 2019

The tire diameter is the deciding factor, not the wheel size. I put 275/60r20 tires on and I was 3-4 miles off at 70, 1-2 around 50. It will need to be recalibrated to be correct.

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newdude · October 31, 2019

3 hours ago, mistermcgoo said:

So of a random post, but I’ve seen this asked in other threads in the past..

I just picked up a 2018 z71 and swapped my replica GMC honeycomb 20” wheels I had on my 2016 LT to this truck. This 2018 came with 20” wheels from the factory, my 2016 came with 17’s originally.

I noticed that my speedo was off by about 3-4 mph at around 70 mph after adding the 20’s. This 2018 is either dead on accurate or off by maybe 1/2 mph at the most..

I know a lot of people have wondered whether or not our trucks come from the factory with a general calibration or a “wheel specific” calibration based on whatever wheel size that particular trim came with originally.

Based on what I’m seeing, it seems to be specific?

What was the tire size on your factory 17" wheels? If it was 265/70R17, that size, the 265/65R18 and the 275/55R20 use the same calibration. Not sure if the 255/70R17 uses the same calibration however. Either way the 275/60R20 you went with is a larger diameter than any of the factory size tires used. GM will only calibrate to a factory tire size that is available so an aftermarket route will have to be pursued.

mistermcgoo · October 31, 2019

9 minutes ago, newdude said:

What was the tire size on your factory 17" wheels? If it was 265/70R17, that size, the 265/65R18 and the 275/55R20 use the same calibration. Not sure if the 255/70R17 uses the same calibration however. Either way the 275/60R20 you went with is a larger diameter than any of the factory size tires used. GM will only calibrate to a factory tire size that is available so an aftermarket route will have to be pursued.

My 2016 with the original 17" wheels had 255/70r17's on them..

So if I'm correctly understanding was you're saying, the calibration on my 16' (may have) had a different factory calibration due to having the 255 width. My current 2018 had 275/55r20's on it originally before I put my wheels with 275/60r20's on.. so it sounds like it (might) be calibrated differently, which would explain why I don't see as much of a speedometer discrepancy with this truck?

mikeyk101 · October 31, 2019

This is a neat tool and it also shows the difference on speedometer when comparing different sized wheels and tires.

https://www.discounttire.com/learn/tire-size-calculator

newdude · October 31, 2019

3 hours ago, mistermcgoo said:

My 2016 with the original 17" wheels had 255/70r17's on them..

So if I'm correctly understanding was you're saying, the calibration on my 16' (may have) had a different factory calibration due to having the 255 width. My current 2018 had 275/55r20's on it originally before I put my wheels with 275/60r20's on.. so it sounds like it (might) be calibrated differently, which would explain why I don't see as much of a speedometer discrepancy with this truck?

You've gone from a 31 inch diameter tire to a 33 inch tire with a 255/70R17 to start. So doing this on a truck with 18s or 20s to start would be less of a gap yes as that is going from about an almost 32" tire to a 33" tire.

Its the diameter of the tire, not the width that the truck is calibrated to. I can have a 265/65R18 on my 2019 LD and that is 31.5" but can keep the speedo identical with a 285/60R18. So an inch wider tire but due to the lower aspect ratio (sidewall height) its the same diameter.