...

*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.