TOM351's answer is very good and the 3.55:1 rear end is generally a good overall compromise recommendation. However, just as you use low gears for accelerating on a bicycle and high gears for cruising, the automatic transmission in the car slects a low gear for acceleration and a high gear for cruising. The basic equation for estimating speed from engine speed and drive train data is:
speed = (tr/168)x(rpm/(tg x rg)
where
vehicle speed in mph
tr = tire radius in inches
rpm = engine revolutions per minute
tg = transmission gear ratio
rg = rear gear ratio
168 = units conversion factor
(This equation assumes no slip in the transmission.)
example
tr = 14
rpm = 2400
tg = 1
rg = 2.8
Speed = 71.4
If you change the tire radius to 17 in the speed would increase to 86.7 for the same rpm. Or for the same speed, the rpm would be 1975.
As a rule of thumb, when you lower rpm at cruise speed, you increase economy but hurt acceleration. You cannot select a tire size that gives you both good acceleration and cruizing economy. That can be acheived with selection of the right transmission and rear end. An over drive transmission combined with a low (high number) rearend is the ticket to get both acceleration and reasonable economy.
|