A few (everything is relative) words on tires and pressures.
Low rolling resistance tires are a specialty item. As is the case in most engineering, design is a series of tradeoffs. Take for example, tire wear. Many of the factors that contribute to long wear in tires do not support some of the other desirable attributes such as traction, wet braking, etc. Similarly, factors contributing to better low rolling resistance performance are at odds with longivity, while retaining traction etc. Some compromise is in order to prevent losing too much ground in any important area of tire performance.
All the manufacturers work with essentially the same materials and laws of nature. It isn't something that you can "fix" at reasonable prices, if at all. It is unrealistic to expect any manufacturer to be able to deliver stellar performance in all rating attribute categories even if their name is Michelin.
Tire pressure: Tires are not neccessarily better because their max safe pressure is higher. Max load rating is important and should always exceed the greatest load ever carried. B U T, that said, it is important to note that the correct tire pressure is not usually the max pressure written on the sidewall. Brief physics lecture follows... (Frenchie, keep me honest.)
To a close approximation, the inflation pressure of a tire multiplied times the area in contact with the ground (footprint) is equal to the weight on that tire. Added up for all 4 tires and you have the loaded weight of the car. Too much pressure and the tire only contacts the road in a small patch at the center of the tread, wearing out the in the center while the edges of the tire remain essentially unused. With pressure too low, the tire tread buckles and most of the weight is carried on the edges of the tire with the center tucking up and away from the road.
Most of us are familiar with the gist of this dissertation and should be able to track the following logic. Somewhere in between way too much and way too little is a zone of pretty good. This is where even tire wear wil be achieved and give longer mileage AND with more tire in contact with the road you will get better traction for steering, accelerating, and braking.
Inflating a tire with a terrific load carrying ability to its maximum pressure when installed on a car that is much lighter than the maximum load will result in serious overinflation. This overinflation is not a hazzard to the tire but to safety and performance. It doesn't overstress the tire but it reduces the contact area and will result in lowered life and poorer traction.
Do not interpret this to mean you should not buy high load rated tires. I prefer tires with a good safety margin of load carrying ability as it implies stronger construction. I DO NOT arbitrarily inflate them to the manufacturer's max rating. It is a MAX rating not a recommendation for any specific application. That MAX pressure is only needed to carry the indicated MAX load. Since you are carrying less you need less pressure for proper tire shape.
Recommendations on the car's door jam sticker or in the manual often give increased pressures for sustained high speed driving. This is because one of the greatest tire killers is heat build up from high speed driving (assuming tires are not run underinflated). The extra pressure is a compromise that reduces tire flex (source of the heating) while operating nearer the upper end of the envelope of proper shape (pretty good zone, mentioned above). I suggest that you will get lower rolling resistance at the high speed pressure and that with only a minor reduction in tread life if you use the high speed pressure for everyday driving. For you "Princess and the Pea" types, you might note a slightly harsher ride. I would not recommend exceeding that high speed pressure for everyday driving.
Perhaps someone well connected with Toyota engineering info will chime in and tell us how far they are pushing the envelope with tire pressure to further reduce rolling friction and increase gas mileage.
As others have noted, the stock tires don't give really long service. That isn't a surprise, tire longivity wasn't the attirbute they were trying to maximize. A good question for those who may have kept really good detailed expense data on their Prii would be: Did buyinig tires that gave longer service (but lower MPG) end up saving any money?
You save on tires at the expense of buying more gasoline but did you save more $ on tire mileage than the increased $ for Gasoline? If yes, and environmental considerations don't sway the decision, then better to buy long life tires and spend more on gas. Or perhaps the ECONOMY of this issue will run the other way and you'll be better off saving the gas and buying tires more often. Any accountant/budget analyst types out there want to share the findings of their detailed records keepiing?????
Pat