So 40 psi == bad idea? (Dealer says so.)

I agree, I have the stock 15's. If I had 17's low profile,I would have to look at the situation differently. I am sure the 17's have a much harsher ride. Which would probably call for a little lower psi.

 

There is very little gain on MPG once beyond 40 psi and also the ride deteriorate quickly.

 

I run 42 in all tires. Not a problem to report to date.

No. There is no evidence via tire wear that they are overinflated.

 

Perhaps it's the tire technology that's changed. I know years ago, overinflation was pretty obvious before 10K miles. It would appear that people don;t have that issue anymore.

 

Toyota recommends that the rear tires have 2 pounds less air, makes car track better I guess.

Correct, different tire design.

 

I think over-inflation also causes the rubber to wear more rapidly due to being stretched.

 

Sounds like a stretch to me!

 

The steel belts under the tread don't stretch much at all.

 

Well I read an article years ago that discussed a process where they made the tread larger than the mold, forced it in, compressed, and they claimed it extended tread life quite considerably.

 

My dealer always sets them to 38 when I take it in. I'm running at 38/36 and have been experimenting with 35/33 all the way to 48/46. The MPG gain above 42 is not as much and the ride sucks. Treadwear on these tires is 800 and the depth was 12/32 new. Max Sidewall PSI is 51.

 

40psi is bad for the dealer. Your tires will last longer so they lose an opportunity to sell you more tires more often.

 

No, you should have told the "mechanic" that BMW requires higher tire pressure for high speed driving.

Yes, tires can burst from over-inflation, but that's closer to 100 psi.

 

I'm not amused at most of the negative comments the 17" wheels receive here. Much of it unfairly perpetuated. Bad roads and speed bumps are more at fault than the "semi low profile" tires on the 17" Prius wheels. Regardless of a "almost" insignificant psi differential.

Second verse, same as the first. Really, 2 psi?

 

My experience in cars is that lower PSI = softer ride but higher rate of tire wear. Higher PSI is better for MPG but = harder ride and better tire wear.

Going 2 psi lower in the rear is largely pointless IMHO unless you're talking an unloaded pickup where there is a bigger difference in weight load front to rear. Most cars and SUVs (even front-wheel drive) don't seem to suffer that much unless you really pull tight turns in an UNLOADED vehicle. If you haul stuff about in the back regularly, it's just easier to keep a 4 tires at the same PSI.

 

But Toyota's manual does say to run 2 pounds less in rears. This must be for a reason.

 

When you're tweaking mpg, I assume everything plays a role, though some certainly more minor than others,. My question is am I supposed to still run 2 lbs less on a trip with a full load in the rear? And then when I get home, am I supposed to let 2 lbs worth of nitrogen out? I don't get my Prius until next week, so I don't even know what the recommended psi is. I've always thought that over-inflating meant less rubber on the road and less control in inclement weather plus greater wear in the middle.

 

^ I think it's mainly to compensate for the car (like most) being front-heavy. I'll often just set all four the same, just so that when I'm rotating them I don't have make adjustments.

 

Given that the front and rear axles of a car rarely bear equal weight, and given that inflation pressure controls the shape of a tire and how it contacts the road, it makes perfect sense that recommended tire pressures should vary between the front to the back.

 

PSI is like voodoo math.

Owning a motorcycle, I know guys who use fancy air gauges ($50+ value) to test air pressure during the day's ride. Desired PSI varies from early morning-cold tire to afternoon-hot tire, and if the tire doesn't have the right PSI, how hard they can push the rubber will vary widely. I just set mine at 42 and enjoy the ride, but I'm not pushing the bike to the max of its capabilities or to any degree where my simplistic approach to tire pressure compromises performance.

If you just drive about in your car, yeah, I guess the recommended PSI is fine with 2 pounds less in the rear, but 2 PSI won't make much of a difference to worry over. Overall, slight over-inflation seems to benefit MPG overall without compromising tire life or wear. If you put the rear under more load, you want more PSI back there. What you are compensating for is your rear suspension. A pickup truck with the bed removed has almost no weight compared to what the springs/shocks attached are rated for. The rear end will bounce all over the place. Under-inflating tires lets the impact energy go into the rubber rather than the axle. In contrast, an under-inflated tire in a pickup loaded with cargo could roll right off the wheel in a tight turn because there isn't enough pressure to ensure the tire remains seated firmly on the rims.

 

DoubleDAZ, if the tire's tread is only making contact in the center then the tire is overinflated. At the pressures we are discussing this is not the case. If you are happy with 34psi in a tire designed for 51psi then be my guest. If the tires wear quickly & unevenly then don't blame the tire.

An underinflated tire will have less traction (as the center is not in contact with the road) which means the car will not handle as well and will hydroplane in less water.