Incorrect speedometer and mileage

[pmikle]

OK, I just became part of "the club", I purchased a 2004 G35 Sedan. I noticed a few days after buying the car that it was running 225/55R17. What I understand is the stock tire is 215/55R17.

I did some driving tests and my mileage is about 10% under (10 actual miles is 9 indicated on my odometer) reported and my speedometer is reporting approx 5% under (actual speed of 66, indiacted in 60).

After some caluculations of circumference the tires now on the car represent only about 1.6% difference from stock. Why a 10% actual differernce? Am I missing something in my calcs?

Stock: 215/55R17 : (17/2)*25.4 + 215*.55 = 334.15 mm Radius = 2098 mm CircumferenceCar has: 225/55R17 : (17/2)*25.4 + 225*.55 = 339.65 mm Radius = 2133 mm Circumference

Granted, the car should have 215/55R17, but what else could be wrong to account for such a big difference between actual and indicated?

[C-Kwik]

Out of curiosity, how did you measure the actual mileage?

[pmikle]

I used three different roads (using roadside mileage markers), and measured a ten mile section (in each direction). Every mile and the entire distance was consistently 10% off.

[Q45tech]

You must use loaded radius [revs per mile] of tire not the mathematical diameter.........tire brands are different you cannot depend on 215 being anything just not a 205 nor a 225 probably close to 215 +- less than 5-9.

10% off should save you 8 cents per mile in depreciation.

[pmikle]

is the loaded revs marked on the tire? so what your saying is the 10% diff could easily be just the tire? i was concerned i had another problem.

i agree on the depreciation, i wasn't going to rush out and get new tires ;-)

thanks,

- paul