Well, steel wheels on steel rails have the least rolling resistance, so in theory the higher the pressure and the stiffer the tire the less energy will go into flexing all that rubber. But there are limits to how much pressure your tire will handle, and higher pressure means a smaller contact patch with the road which means longer stopping distances in an emergency. Not to mention uneven tire wear. I've run my tires around 3 psi over the manufacturer's recommendations for decades. I really have no idea how much or if it actually does help fuel economy, but for me it seems like a decent compromise. I've never had any uneven wear issues or difficulty stopping, so it seems about right to me.
Every pressure setting involves compromise. Personally I slightly override Toyota's recommendation, but not by much. I've found little difference in mpg as I've raised pressure. I suspect too that higher pressures will accelerate suspension wear.
Toyota's recommended pressure is a compromize between ride comfort and MPG. The "optimum" pressure is a function of driver preference and the Brand/Model tire on your Prius. I run my Nokian Wtires (2004) at 42(F)and 40(R). JeffD
So far as I know, there is no upper limit to MPG optimization, but I want to convince you NOT to optimize for MPG. Lets assume Toyota has optimized for passenger comfort, any change we make will reduce comfort. A great many drivers find that Toyota's setting is under inflated for ideal tire wear, the edges wear faster than the center. A pressure somewhere near 40 PSI in Front and 38 PSI in the rear is best for even tire wear. if the entire tread surface is wearing evenly, then it is true the the entire tread surface is gripping equally. This is the ideal I would like you to shoot for. The tires will last the longest, and so save you money, and grip best potentially saving your life. Could you save another 2 MPG? Yes but at what cost? These tires are exaggerated.
Post # 2 has some good comments. Not sure about optimum..... Higher pressure will reduce the tire contact patch and increase miles per gallon from less rolling resistance. However, that smaller contact patch will also reduce braking distance (safety issue to me). Additionally higher pressure will increase the wear rate of the center of the tire. Corner handling and overall grip may also be compromised. Lastly, higher pressure typically reduces ride comfort. Optimum could be referred to tire temperature. Find the tires operating temp range (mostly likely by contacting the tire manufacturer) and increase pressure to stay with in that temp range. A Pyrometer will be required to set this optimization. Again to post 2, bumping the tire pressure up by 3 psi cold seems a very reasonable option to eek out more miles per gallon. A cost of gas/mileage increase vs cost of tires comparison table might also uncover optimum. Good luck.
