Tire Size Tuning Question

[Rob Bell]

OK so I took my car in to the dealership today to get the speedometer calibrated. These people are helpless. Their equipment can only change to factory-approved options. I'm calling around to speed shops to see if someone can do this.

A while ago I put 245/50 R16 tires on the car. The stock ones were 235/55's of course, and I have 3.42's. So I'm thinking that to calculate the new gear ratio I need to take the new tire size (245*.5 = 122.5) and the old tire size (235*.55 - 129.25) and then divide them (129.25/122.5 = 1.055) which means the new tires are making the speedometer 5.5% faster than it should be. So then, if I take 3.42*1.055 = 3.608 which would make my new gear ratio 3.61 to correct the speedometer. I have verified that the speedometer is indeed about 5% off by timing myself against mile markers.

So, can HP Tuners or the Hypertech programmer actually allow you to input a calculated gear ratio like this? Or does the car's computer only give you options to select from?

[ssms5411]

Re: Tire Size Tuning Question

Hptuners you you just divied you tire diameter by your gear ratio, like mine with 275/40/17's diameter like 25.5(guesssing) and 3.73's its like 6.8??? and hptuners will set up the shifts for you if auto and thats all you have to do is put in the 6.8?? number. What year car ?

[PalmHarbor]

Re: Tire Size Tuning Question

If you can borrow any Garmin GPS, it has a function that tells you the speed that
the car is moving at. It is dead accurate. That will tell you what speed the car is
going v. the camaro speedo.

[J Phillies]

Re: Tire Size Tuning Question

the new tires should only be 2% off



this link may be useful

http://www.miata.net/garage/tirecalc.html

[bigbrian442]

Re: Tire Size Tuning Question

I believe GM uses the same size in the PCM for the 3 sizes of tires that came on the 98 and later F body. The 235 and 245 16"s and the 275 17's. The speedo almost never matches what the PCM reads when your scanning perfectly.

[Rob Bell]

Re: Tire Size Tuning Question

OK I'm back, I was gone all weekend. I saw that Miata calculator and noticed that it stated I should be 2% off, which I thought interesting, because when I run the numbers in the real world it comes out to 5%. So I'm not sure what to think there. The guy at the dealer said that their computers only allow them to make changes that would have come stock from the factory. So even though TA's and Formulas may have had the 245/50 R16 tires, I believe they had 3.23 gears correct? So, 3.42's and 245/50 R16 tires were not a factory option. So they cannot make that change. Is that stupid or what? I guess GM does that to prevent them from screwing anything up too bad. I like the idea of the GPS. I don't know anybody with one though. I'm going to call around some more to see if any local shops can tune my PCM. If not, I'll be saving for HP Tuners. Having an inaccurate speedo will drive me nuts.

By the way, my car is a 2002.

Another interesting factoid is that the dealer cannot tune for tire size changes on F-bodies, only gear changes. Later model GM cars can tune specifically for tire sizes.