How much of an increase in mpg can I expect with overinflated tires?

[theclencher]

Originally Posted by Ernie Rogers

Let's try some numbers. A 195 wide tire is 195 /25.4 = 7.68 inches wide at the tread. The load on one tire is about 900 pounds. At 40 psi, the area of the contact patch is--

A = 900 lb /40 psi = 22.5 square inches.

Divide that by the tread width to get the other dimension--

22.5 /7.68 = 2.93 inches long.

To make 2.93 inches of curve flat on a 25 inch diameter tire, you have to deflect the center of it by 0.11 inches.

The harder it is to get the tread surface to move in that amount, the more energy the car loses as it rolls. That means soft rubber is better, and a thin tire is better. "Low friction" rubber and tire construction is also better, and that's what you are paying for in an "energy tire."

Ernie Rogers

A 195- tire is not 195 mm wide at the tread- the designation is for max sidewall width (section width). That throws all yer numbers off but the rest is good I think... xept the phrasiology of "the harder it is to get the tread surface to move..." isn't quite right, as overinflating will do that yet lower r.r.. More accurately, if psi is ignored and we consider how much energy the rubber in the tire structure absorbs (hysteresis) THAT is what is soaking up the energy. That is where rib treads are better than tread blocks (less flexing), thinner is better than thick (less energy absorbtion), worn out is better than new (less flexing), etc.

http://www.tirerack.com/tires/tirete...e.jsp?techid=7

P.S. A little something on sidewall psi ratings:

"It is important to note that the maximum load is rated at an industry specified tire inflation pressure that is often lower then the tire's absolute maximum tire pressure. The tire pressures used to determine the maximum load the tire is rated to carry is based on the sizing system industry standards applied to the tire.

Sizing System Tire Load Range Load Pressure
P-metric Light Load
Standard Load
Extra Load 35 psi
35 psi
41 psi
Euro-metric Standard
Reinforced or Extra Load
36 psi
42 psi


However, the tire's maximum inflation pressure may be greater, such as 300 kPa (44 psi) in this example or even 350 kPa (51 psi). This is done to accommodate the vehicle manufacturers desire to tune the tires' high-speed capability, handling qualities and/or rolling resistance to better suit the vehicle."

http://www.tirerack.com/tires/tirete....jsp?techid=21

Looksta me like there is plenty of latitude for interpreting sidewall "max psi"