[SuperTrooper]

My, that was a fascinating morning of reading. Thank goodness I'm at work!

I learned a lot, but nothing that would indicate that over-inflation offers SIGNIFICANT increases in MPG. The main fact behind my conclusion: at least 3 studies agree that tire rolling resistance (RR) is the source of 25% of energy consumption for a vehicle moving at 100kph/62mph. You have to make LARGE gains in RR to reduce total energy consumption (i.e. MPG). All things being equal, you have to reduce RR 4% to make a 1% improvement in MPG. Since MetroMPG's Metro gets significantly better milage than most it should see the larger gain: about 0.7 MPG. Most wouldn't see half that.

Then you have to factor in the other important things related to tires: accelerating, braking, turning. (I'll just call it handling.) The Dutch study proved the negative impact of over-inflation on handling, enough that they deemed it significant enough to overshadow any benefit to over-inflation. Surprisingly, they found wet road performance even further degraded by over-inflation, which is contrary to what other posters here have been known to say.

Other tidbits I learned:

RR increases singificantly at speeds above 60 MPH. Another reason why slowing down is important.

Warmer tires have a lower RR. No kidding. The point being that in winter conditions some tires never get warm enough. Surely a factor in lower MPG often seen in winter.

Lots of talk about critical pressure increases. Air expands at a given ratio to temperature. Over-inflated tires reach critical temps sooner, but since expansion is limited more heat is retained. If you over-inflate you MUST slow down. If I were going to over-inflate I would change over to nitrogen since the expansion ratios are lower.

According to the Finnish study one way to reduce RR is to go to a higher circumference (taller), lower aspect ratio (short sidewall) tire/wheel combo.

The "spring rate" of air in the tires is a critical factor involved when the factory chooses a tire. It's part of the whole load/pressure/heat formula. This is why when you go +1 or more on wheel size you MUST widen the tire so that the volume of air in the tire remains essentially the same. Otherwise, the load increases, which increases temp, yada yada.

I've also been googling trying to find DATA to support the claims. All I find are anecdotal claims on sites similar to this one. Nothing scientific.

I'm still skeptical.