<VV> Tire diameter and mileage

[walt matenkosky]

Just a couple of thoughts here. Remember that for all intents and purposes, 
you are not going to change your engine or final drive ratio. Let's consider 
that we are going to drive at the same engine speed for the sake of 
comparison.

If you increase the diameter of the wheel/tire combo, then you increase the 
true distance travelled by the difference in diameter times the number of 
wheel revolutions. Let's say you put on a tire that is 10% larger than OEM, 
then you will travel 10% farther. Your gas mileage will be better in that 
you will have travelled farther in the same number of engine revolutions. 
There is a penalty, however: your acceleration will suffer because it will 
effectively numerically decrease your final drive. If you have a 3.60 axle, 
it will feel like a 3.10.

It is a popular way to change gearing for autocross courses. Going the other 
way, to a smaller wheel & tire, effectively increase your final drive, 
making that same 3:60 feel like a 4.10. Greater acceleration, worse mileage.

As far as speedometer error, you can estimate the difference based on change 
from OEM diameter, but there are other factors involved, so the best thing 
to do is to go out to a measured course, drive it at a constant speed as 
indicated on the speedo, and measure the time. Compute miles/minutes and you 
can find the true difference AKA correction factor you need to make in the 
speedo drive gear, speedo head, or in your head. I used a conversion factor 
to make a speed chart based on engine speed in gear until I could correct 
the speedo.

Walt