Tire Pressure

First off, you won't break anything by running at 42/40. The possible downside is a harsher ride, more buzz/rattles, and maybe uneven tire wear.

The upside is better fuel mileage, and possible more responsive handling

 

I'm not an expert on tires and pressure, but, the posted maximum cold tire pressure is indeed 44 psi on the stock tires.

People can do whatever they want, but I just think 42 psi in the front is getting awfully close to that 44 maximum figure.

Toyota says 35 psi front. You've mentioned 42. You can try 38 front, or different settings until your comfortable.

Anyrate, again, I wouldn't feel comfortable going near that maximum figure. I'm pretty darn happy getting 55 mpg while set 3 psi over recommended and the ride is taught enough for me.

 

Why?

With my last set, I drove over 77,000 miles that way.

Wear was actually greater on the edges and there were no problems whatsoever. They are obviously designed for it, as stated on side of the tire itself. What additional proof do you need to feel comfortable?

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I REALIZE that there are many folks here that run 42+ psi's in their tires. 44 PSI is the maximum posted psi in the stock integrity's. so here is my 2 cents. 42 psi = 45+ when the temp is hot and you've been driving for a while.

SO,,,,,,,,,,,,,,,,is it safe to go over the rated maximum psi's? many have and are successful in achieving good mpg. knowing that 42 psi can easily surpass the maximum rated psi in the tires, i would stick with 40/38. what do i have in MY OWN RIDE? 42/40. My DAUGHTERS prius: 40/38

It's up to you and what you are comfortable with.

 

As a former radial tire development engineer with General Tire, I can tell you that keeping your tires at the cold tire inflation pressure (CTIP) listed on the sidewall is safe. We never would have put the number on there if it wasn't. Even at the cold tire inflation pressure you have a window of 150% over to cover in the event you went from temperatures of say 32 degrees and then drove to Florida and 80 degrees. Also, your roll rate resistance will be lower at CTIP, and your tire wear rating is actually based on testing at the CTIP. Car manufacturers lower the pressure for ride purposes, not safety. The most unsafe condition you can have on a tire is underinflation. Tires build a great deal of heat when they can over flex as they do underinflated. Heat kills tires. So, bottom line, you can take the tires to CTIP safely. I have always had mine that way ever since I worked for a tire manufacturer. As had been said however, your ride will be harder, but not unsafe.

Good luck!

 

For those of you worried about Toyota dealers telling you to run the tire pressures listed by toyota, it was back in 04 when i got my first Prius and had some questions, the Prius tech at our local Toyota dealer TOLD me to use 42/40 otherwise I'd be in later complainging about how quick the tires wore out.

 

See my previous post, but to comment to your comment, your tires can easily go over 50 psi on a hot day. Is it unsafe? No, IF you started out at cold tire inflation pressure (CTIP). Tire wear ratings, traction ratings, temperature ratings and tire load ratings are all based on testing at CTIP. We use the CTIP as the reference pressure for all the ratings a tire has. Lower pressure means less load capacity, shorter tread life, and lower traction (yup, lower because the sidewall flexes a lot more). So when your tire doesn't get close to that 40,000 miles that was claimed for it, chances are you ran it at much lower pressures than CTIP. On top of that, if you kept the car loaded your actual load rating at lower pressures was significantly lower. The closer to 100% of load, the faster those tires wears.

 

I was pretty religious about keeping my OEM tires at a CTIP of 42 psi for over a year, then I found out that the gage I was using (a $10 Victor dial gage) was consistently reading 10-15 psi under the actual pressure. So I drove for over a year with my tires actually around 50-55 psi. After 15k miles, my tires are showing a lot of wear, needing replacement soon. Oddly, the wear isn't in the middle where I would expect with overpressure, but is most severe on the sides of the front tires. Probably an alignment issue.

So, I'm living proof that you can drive around with significantly over-inflated tires and not have any significant problems. Sure, I have a harsh ride, but I've always attributed that to the absolutely horrendous San Diego pavement conditions.

 

You aren't. They almost always do exceed that value when hot. It's taken into account already.

That's why the tires say "Cold Maximum" for PSI.

Overinflating is only when you have PSI greater than that when measured cold (before you drive).

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I've driven my '08 for the last 14K miles with 40 in Front and 38 Rear...no ugly tire wear or significant harsh ride experienced. The only thing that I got is great mpg every tank.

 

70,000 miles at 40/38 (more or less), no tire issues. The originals lasted over 67,000 and still had legal tread. My lifetime mpgs are just short of 50. You will probably get longer tire life and more mpgs, by higher than recommended tp. I can't think of a down side. The ride is just fine for me. If 42/40 seems too high back it down a little. Its not really that big of a deal.

 

I'm running my original tires at 60/58. Maybe I should be worried - but I'm not. I hoping to get a lot of miles out of these tires too.

 

I wouldn't worry about being worried. I know someone who is at 105K+ on the original tires that came on their '05 Accord and the tires are still above legal tread depth. They are running 60-65 psi. So, when running higher psi, choose your tires carefully because you may have them for a looong time.

 

Tell me your opinion diamond on this - I just bought a set of Nokian Hakk R. It is a studless winter ice and snow tire. Its one of the fancy new type with soft tread rubber compounds. Its expensive. Supposedly a long tread life tire and low rolling resistance - at least when compared to similar tires. Nokian makes a big point about how it is important to drive these tires only in the winter so you don't destroy the tread life in hot weather and dry roads. The max sideway pressure is 51 Lbs.

If I use high pressure - like 50 or 60 Lbs - will it affect the tread life negatively?

If I use high pressure will it affect the performance of this tire on snow negatively? Ice?

 

I picked up a tread depth guage to see if I'm under or overinflated.

 

Running above max cold rated pressure (the value shown on the sidewall) incurs some risk of catastrophic blowout. To me that is not worth a little more MPG.

 

If that is true that I agree and I'll change my ways. Is there any data showing the extent of that risk at different pressures? I assume - without knowing - that there is some risk of catastrophic blow out even at pressures within the rated pressure. I'm interested whether and to what extent that risk increases as the pressure increases over the rated pressure.

 

rpg51,

As long as you inflate the tires cold to the maximum shown on the sidewall you are safe. The soft tread compound gives you improved traction over all-weather tread because at the same temperature, the soft tread is, well, softer than the all-weather tread. However, as Nokian warns, you don't want to run them in Florida during the summer. Think of drag cars, and race track cars. They use a softer tread to get great traction on the track, but as far as mileage, they don't have it. Hence why you see them getting changed once or twice during a race. The Nokians in the summer would simply wear out rapidly on hot pavement, much like a race tire. Your car tires have a much harder tread compound, but the trade off is lower traction. Not an unsafe reduction as you are unlikely going to drive your Prius at race car speeds.

Also, there is ample safety margin built into all tires to account for wide temperature swings.

 

Thanks for those responses guys. Two questions remain though -

How does ice and snow performance of soft tread winter tires vary with pressure? Does it change depending on whether I have them at the Toyota pressure numbers or the sidewall pressure numbers (these Nokian's are rated at 51 by Nokian).

And

Is there any data out there on the affect of pressures higher than rated on risk of catastrophic blow out? The only studies I have ever seen relate to dangers of low pressure, not high pressure.