I know on the driver side panel, there is a listing of what the tire pressures should be, but looking on the MPG thread, I am seeing people with 'higher' tire pressures than the ideal. Of course, I am running on the stock Prius v tires, (in this case, the ones that came with the Prius v 5, 17" rims) so I was curious what is the 'ideal' tire pressures one can hope for to improve MPG based on stock tires.
What is the max pressure stamped on the tire sidewall? If it was me, I'd run the front tires at that pressure and the rear tires 2 lb less for maximum MPG results.
The Tire on the left is under inflated, it will wear out on the outside edges, it has less contact with the road than the tire on the right, so will stop and steer worse. It may well feel most comfortable, as it can absorb bumps due to low pressures. The Center tire is over inflated, it will wear out in the center, it also has less contact with the road that the tire on the right, so will stop and steer worse. Ride quality will also be worse. While fuel economy may be improved, I do not recommend over inflating. The tire on the right maximizes stopping and turning with very good fuel economy and not much degradation of comfort. Sadly, the 'ideal' varies by tire, not just by Pounds per Square Inch. My Goodyear Integritys wore near the edges until I ran them at 42f/40r, but same sized Goodyear Authority Fuel Max tires wore out in the center even at 38f/36r. They just did not like as much PSI. Someone watching tire wear is the only true test, but I would start at 40f/38r and watch for wear. (These tire are not in the same order)
You 17" tires are likely rated at 51psi so like Jimbo said, start at around low 40s and work your way up from there. The ride will probably be too harsh long before you get to sidewall max.
I used 45 to 48 psi for the entire life (about 42K miles down to 3/32) of the OEM Toyo tires on my Prius (size 215/45-17). The tires still wear a bit more on the outside edges than they do in the center due to cornering forces. I think the belt under the tread of well designed radial ply tires is stiff enough to prevent the wear patterns due to over/under inflation shown in Jimbo's post above, providing you do not exceed the pressure on the sidewall or go under the vehicle manufacturer's recommendation. The booklet I got with my last set of Michelins says always stay between those two pressures. I do not think there is any one best pressure, it will depend on weather, what kind of roads you drive on, vehicle weight, the specific kind of tire you have, and a lot of other things. The pressure listed in the owner's manual and the driver's door frame is an estimate by the vehicle manufacturer and is usually biased toward a soft ride. In general higher pressures will provide better fuel mileage, more resistance to hydroplaning, and more precise steering. Lower pressures provide a better ride and slightly better traction on dry pavement.
Do not use the pressure listed on the sidewall for anything but reference for max load. The correct pressure will be found by starting with the pressure listed for your car on the door jam. After setting the pressure with a good digital gage, drive the car at highway speed for 20 or 30 miles. When you are done with the warmup ride, check the pressure (while tires are hot). The pressure should be within 10 percent of the cold reading. I prefer a more firm ride and increased gas mileage provided by having the tires inflated so that I get a 5% increase in pressure. This will give you the best performance, best life and good gas mileage. Also, it isn't dangerous like gross over inflation.
Gross over inflation is when the tire is no linger doing the job of putting the correct amount of tread on the pavement, it is so stiff that it does not absorb road shock and transfers it to the wheels, suspension and chassis, puts extra strain on the wheels, exceeds manufactures limits for achieving the load rating, etc. A tire that is rated for some number like 1500 lbs at 51 PSI will not work or ride properly with 800 lbs of load and 51 PSI.
We have the same model w/17" tires....have 4500 miles and have been running 40 frt/38rear at 60* outside. Ride is acceptable to me and I feel better protection for the low-profile tires....
I have 40PSI for front and rear. Been getting great MPG 57 for the last 3 fill ups. Good ride as well.
