Changed my tire size, now how do I recalibrate the speedo??

[dmauch]

I went from the factory 265/70/R17s up to 275/55/R20s on my 2004 - is it possible to reprogram the vehicles computer so that my speedo, fuel economy and mileage are accurate?? Do you have to replace the speedo gear? I had to do that on an old jeep I had...

Also, what is the correct torque for the lugs on a 2004 with 20" wheels?

Thanks guys!

[Foxriverkid]

Since knowing how fast you are going is of paramount importance, I would suggest going to AAA or police approved speedo shop. They will do it for you and it's not expensive. Let your fingers do the walkin' and look in the yellow pages.

[dmauch]

Can they do it at the dealership? Does anyone know what it involves such as just a quick computer reprogram or is a new speedo gear required? The majority of people on this forum have aftermarket wheels and larger tires so I know this must come into play a lot. What does everybody do?

[hcvone]

Originally Posted by dmauch

I went from the factory 265/70/R17s up to 275/55/R20s on my 2004 - is it possible to reprogram the vehicles computer so that my speedo, fuel economy and mileage are accurate?? Do you have to replace the speedo gear? I had to do that on an old jeep I had...

Also, what is the correct torque for the lugs on a 2004 with 20" wheels?

Thanks guys!

Your stock tires were 31.60" high, your new tires are 31.90" high, or .095% different, not enough to worry about, when you are going 60 mph you will really be doing 59.4 mph, you would need to redo the speedo when there is a 3% difference, you are less than 1%. The stock torque wheel setting is 140 ft. lbs.

[ewill3rd]

The only way we can program them at the dealership level is to call GM and tell them the VIN and the tire size you installed.
IF, and ONLY IF, it is a GM APPROVED tire size they will give us a VCI number from engineering to download and program into the vehicle.
The EBCM also needs to be updated with the proper tire size.
If the tire size is not an approved size by GM they will pretty much hang up on us. I think they charge us money for a VCI number on the order of $200.
I'd have to check to be sure about that part.

[dkozloski]

Originally Posted by ewill3rd

The only way we can program them at the dealership level is to call GM and tell them the VIN and the tire size you installed.
IF, and ONLY IF, it is a GM APPROVED tire size they will give us a VCI number from engineering to download and program into the vehicle.
The EBCM also needs to be updated with the proper tire size.
If the tire size is not an approved size by GM they will pretty much hang up on us. I think they charge us money for a VCI number on the order of $200.
I'd have to check to be sure about that part.

ewill3rd, that's what I've been talking about for a long time. The problem is that there is a whole raft of stuff being calculated in the computers that has to be accounted for. A VCI number has to be compiled for each and every combination of components. My understanding is that a one-off VCI number is VERY expensive.

[hcvone]

Also most hand held computer programmers will change your speedo to the tires you are using, in this case nothing needs to be done.

[caddycruiser]

As was said above, you just went from one stock size to another, and the overall diameters are virtually identical--hence, why both of those tire sizes were chosen for the OEM 17's and 20's.

So don't worry about it, you're fine

[cadymon]

take it to the dealer

[dmauch]

Excellent...thanks for all of the info guys!

Dan

[ewill3rd]

Using a tire size calculator it does appear that your rolling circumference is close enough that you might not need to worry about it.
The computer counts rotations and uses the stored tire size and gear ratios to determine distance. The rolling circumference is what is critical, but you didn't change that much.

[slk230mb]

Doesn't tire wear essentially cause the same thing to happen?

[ewill3rd]

Technically, yes.