The following chart of Tire sizes and axle gear
ratios are meant to give a general idea of the relationship between the
differential gear ratio and the tire size and what effect the ratio has
on power output and fuel economy. The chart is based on RPM levels
at 65 mph (105 kph) and a drive
train running at a 1:1 gearing ratio, meaning the transmission and
transfercase are passing power straight through at a one to one ratio without reducing or
overdriving the engines rotations per minute before it reaches the differential.
It is also worth noting that a manual transmission will be the most
efficient at transferring power through to the axles and that an
automatic transmission will have some inherent slippage without a
locking torque converter. Also with an overdrive transmission,
your engine RPM will be approximately 20 to 30% less than shown here
depending on the overdrive ratio.