Rodan

"Donut" wrote

.....if underinflated tires lower gas mileage .. how does it
follow that overinflated tires also decrease gas mileage?
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pkurtz wrote:

....overinflation reduces gas mileage because it reduces
the footprint of the tire on the road and contributes
to slip. The more slip, the worse mileage. If you
overinflate your drive tires, especially on the light
weight of a truck rear, slip increases exponentially.
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Friction physics illustrates that slippage is independent of
the footprint area of the materials (pavement and rubber).
That is: F = U x N where F is the frictional force, N is the
downward force on the tire and U is the friction coefficient,
probably about 0.4 for pavement and rubber. Contact
area is not a factor, because as contact area is reduced,
the unit pressure (tire force divided by footprint) increases
inversely to keep friction constant.

Almost all the energy loss in tires comes not from slippage,
but from flexing in the tire as it rolls. The wasted energy
appears as heat you can feel by touching the tire instead
of as power for the car. The higher the pressure, the higher
the mileage. This is true for overinflation pressures as well,
so even dangerous overinflation will improve gas mileage, at
the risk of early wearout (from distorted treads) or blowout.

Rodan.
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