Tire sizes/Speedo correction?

Marc Russ · November 18, 2016

Hi All,

I see some guys going a taller than stock tire. As this obviously changes the accuracy of the speedometer I was wondering what everyone is using to correct the discrepancy or do they even bother?

Also I see some people going to 18" wheels instead of the stock 20's. Any advantages? Better wheel selection/price?

Thanks,

Mark Mortenson · November 18, 2016

I went from 18" s to 20"s and thought the ride would change for the worse but it didn't. Running 285 60 20's now and in my opinion the stability improved with less sidewall to flex. I'll check with my gps on the speedo but wasn't concerned with it being off or not.

480Ruger · November 21, 2016

I went by the dealership and asked Friday. They said unless the tire size is in the system then no. So how do they change the lifted trucks with 35"/315s? I know you can use an aftermarket tuner but....

Black_Betty · December 9, 2016

I used an aftermarket tuner for mine

SlaxhX2500 · December 23, 2016

I used an aftermarket tuner for mine

Which programmer did you use. And will that void your warranty?

revrnd · December 23, 2016

Which programmer did you use. And will that void your warranty?

Someone posted recently that there is a "programmer" that more or less is a VSS corrector. It doesn't involve the other controls. Not sure of the name, but it was posted that they go for around 200 bucks.

Black_Betty · January 15, 2017

Which programmer did you use. And will that void your warranty?

I used the Hybertech Speedometer Calibrator. No it did not void the warranty. I contacted the dealership first before I did it. They sold it with the lift on it but after a few days of driving it around I noticed that the speedometer was off a little bit (5-6mph) when going 45-50 mph.

Big Whiskey · January 31, 2017

Lots of good advice here. Yes, the dealership can re-program the speedometer, but only to available factory sizes that are in the computer system. I had them do that with my truck, when I switched to 34" tires. They basically told the computer that I'm running the largest factory size available. It's still 1-2 mph off at 60 mph, and 2-3 off at 70 mph, but that's closer than if it were still programmed for the dinky little 265's that came on it from the factory. And yes, many of the dealerships that sell lifted trucks use an aftermarket speedometer calibrator, which shouldn't void your warranty because they don't change anything else with the engine or transmission.

all4glh · February 1, 2017

You can do it with HPtuners. It will only cost two credits ($100) to do a specific VIN, unless the tuner unlocked the whole year in which case they will determine the cost. Then all you need is the tire diameter. All in all, takes a few minutes.

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hiltz · February 18, 2017

I contacted GM directly and they told me the Hypertech will void warranty... reason: GM does not know if it affects any other modules that can lead to failure of components.

KA0S · February 18, 2017

I contacted GM directly and they told me the Hypertech will void warranty... reason: GM does not know if it affects any other modules that can lead to failure of components.

Which warranty is it going to void??

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hiltz · February 18, 2017

Which warranty is it going to void??

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The guy basically said if anything goes wrong with tranny, ABS, RPM module... yadda yadda... GM will not cover the repair. He said the controller that changes the tire size in computer will most likely also change various other firmware within modules in conjunction with rpm, shifts and so on which can be related to failures. so I'd say powertrain/ECU

KA0S · February 18, 2017

The guy basically said if anything goes wrong with tranny, ABS, RPM module... yadda yadda... GM will not cover the repair. He said the controller that changes the tire size in computer will most likely also change various other firmware within modules in conjunction with rpm, shifts and so on which can be related to failures. so I'd say powertrain/ECU

This sounds like they're slightly full of it. It's a bit far fetched to say that it would affect all of those items. Considering they do the same thing with other sizes pre-loaded into their system.

Personally, if it's only to adjust your speed 1-3 mph, I wouldn't even worry about having it done. I don't know enough about running tire/rim combos outside of OEM, so I've never had to do it. I always increase the sidewall if I drop rim size, so I'm within the same spec as what the vehicles came with.

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