In a message dated 99-03-31 22:46:10 EST, Horace K. Sawyer wrote:

> The idea being to lower my final drive ratio a bit (is this right
>  terminology?) and make the motor turn less revs on the hwy, burn less fuel,
>  etc.

The fuel consumption is not only dependent on engine speed, but on throttle
position.  It takes a fixed amount of work fo push your van a certain speed.
This amount of work doesn't change with engine speed or tire size.  It you
have the engine run at fewer RPMs, each revolution will require moe gas and
you'll supply more gas by pressing the accelerator pedal down a bit further
than before.  If you like to travel particularly fast and you find your RPMs
consistently out of the tachometer's green "happy zone" then a increase in
tire size might be useful at allowing the engine to run at a more normal
speed.

Your gas mileage wouldn't change significantly though.  However, your
"percieved" gas mileage will likely decrease.  I say percieved, since the
odometer doesn't actually measure distance, it measures tire revolutions.
Each turn of the wheel moves the odometer a certain fixed amount.  Large or
small wheels both turn the odometer the same amount for each revolution.  If
you use smaller than standard wheels, the distance that each tire rolls will
be less, but still counted as one revolution by the odometer and your
"percieved" fuel mileage will increase, but your actual mileage will likely
be very similar to other sized tires after correcting for tire size.

A detrement to larger size tires will be slower acceleration, slower travel
up grades, and an increased need to downshift whil traveling up grades.  If
you imagine ridiculusly small tires, like 2" diameter, your engine could just
spin them and not move the van forward.  At the other extreme, with 20' tall
tires your van would probably not have enough power to turn them, like a bolt
that's too tight to turn with a small wrench. The slower acceleration is due
to essentially giving the road a longer lever arm (your tire's radius) to
resist your acceleration.  The slower speed up grades and need to downshift
are a result of running the engine at lower RPMs, below the engine's maximum
torque and horsepower speeds producing less than optimal power and requiring
a downshift to get the RPMs higher into a greater torque range.

Regards,
Jim Davis