...
ignore the actual equations for the moment and just follow the "what goes UP" and what goes DOWN" discussion:
MPH = ( 60 / (G × A))( RPM / rpm )
where:
MPH = vehicle speed, miles-per-hour
RPM = engine speed, revolutions-per-minute
rpm = tire speed, revolutions-per-
mile
60 = conversion constant, minutes-per-hour
G = transmission GEAR ratio, ie: 3.21:1, etc.
A = AXLE ratio, ie: 3.08:1, etc.
Any values
above the vinculum (divisor line "/") are
GREEN values; any values
below the vinculum (divisor line "/") are
RED values:
•
increasing any
GREEN value
increases MPH.
•
increasing any
RED value
decreases MPH.
•
decreasing any
RED value
increases MPH.
•
decreasing any
GREEN value
decreases MPH.
...and, tire/wheel
diameter affects
rpm:
rpm = 5280' / ((dia" × PI)/12)
The larger the tire diameter, the fewer rpm's needed per mile and, conversely, the smaller the tire diameter, the more rpm's needed per mile. Thus, tire/wheel diameter is a GREEN value (see here: http://www.gordon-glasgow.org/tirecalc.html ):
dia" = (5280' × 12)/(rpm × PI)
Until the point where you "lug" the engine, anything that enables you to go faster (increasing MPH) enhances fuel economy. This is why "race" cars have 'deep' (higher numbers) transmission gears and axle-ratios while "economy" cars have 'lighter' (lower numbers) transmission gears and axle-ratios.