Quick question, here's my scenario:

I bought a 2001 Z28 M6 on Tuesday from a local seller. The car has 54,000 miles on it. The owner gave me all of his service receipts and receipts for parts he purchased. He had a Hypertech programmer that he used for the 160 stat and speedo recalibration (the car has 18" Z06 rims and 275/35/18 tires).

So, I am looking through the Hypertech manual and it says you can adjust the gear ratio (not literally, simply what the computer reads) to the ratio of choice. I noticed in his paperwork, he had it set at 3.23. I know M6's come with 3.42's - so my mind started wandering . . . did he set the gear ratio lower to make the odometer read slower?

I went back through all his invoices, and on average, he put 1,000 miles per month on the car during 4 years of ownership. He purchased the programmer in 2004 and I'm not sure how often he ran it on the car (I would assume all the time, since he put a 160 stat on it, and the programmer allows for fan adjustments).

My questions are:

1) Should I be worried that setting the gear ratio at 3.23 threw the odometer off by a lot?
2) According to my calculations, going from a 245/50/16 to a 275/35/18 would have zero affect on the effective gear ratio (both tires have diameters of 25.6 inches). So in all realilty, he didn't need to change the gear ratio. Or am I missing something?

Thank you!

 

The tires are basically the same diameter, so no correction was needed. Setting the PCM for 3.23 gears, when the actual gears are 3.42 will cause a 5.8% error in the speedo and odometer. On a car being driven 12,000 miles per year, thats a 705 mile error.

If you think about it, with the PCM set for 3.23 rather than the true 3.42, for each rev of the DS the PCM will think the car went farther than it really did (distance travelled per DS rev = diameter of tire x pi / rear axle ratio). Your odo was racking up 12705 miles for each 12000 miles driven.

Brain hurts.........

 

Thanks a ton! I was worried the % would be more than 5-6%. So, assuming he drove with the program loaded the entire time, my odometer could be off by 1,500 miles. Not bad.

Its actually amazing they sell programmers that allow people to mess with the odometer like that. I know its a federal crime to tamper one or use a product to alter the odometer. But for $400 anybody can do it.

 

Any chance the rear end gears were actually changed?

 

I'd have never even thought about your concern! I'd only be thinking about the inaccurate MPH
Perhaps he just made a plain mistake when putting in the ratio..
Anyway - thats still low miles and if sold any buyer would understand the few mile difference - at least it's higher, not lower than actual.

 

Great now everybody is going to program there there car's to 4.56's or lower. I wondering if that screw's up the shift points or converter lockup

 

Couldn't you set the shift points as the right points then set the gear ratio something like 4.56s and just guestemate your mph based on the rpms?