A question about tire pressures, please...

Hello,
I am in same situation. Bought PiP few weeks back.
When they sold, it was inflated to 33PSI front,30 PSI rare.
Now I have fixed it to 35PSI and 32PSI,but 'scared' to inflate further.
Is there any risk doing so , I live in CA, drive in Bayarea.

 

At the higher tire pressures you are giving up a little bit of stopping distance, particularly on wet roads. If you are hauling your kids in the back seat you might want to give up a little bit of mpg and retain a bit more control.

 

Yes Julia that is the usual information on tires. Yet it's about 2/3 of the inflation the tire can stand. That means even you inflate to 55-60 lbs,the tire can still stand but not for running. Under 45 lbs will be safe to do so. However that makes your ride very stiff though you can save some gas. Official for prius will be 35 front and 32 rear(Gen II). See if you can stand the stiffer ride at the present new inflation. If OK, keep it. Otherwise back to official figure.

 

Please tell us the name and model number of the tire. On my car I run Kumho's KR21 as an example.

 

Overinflated tires give a great boost to MPG. Yet if you drive on lots of rough roads, especially gravel roads, the higher you go in tire pressure the more you'll start to lose traction, as well as be susceptible to flat tires.

What's most important for the safety of your child is your ability to sense changes in tire traction in everyday driving conditions. And a good way to learn how to do this is by experimenting with how tire traction feels at different tire pressure levels

How I learned it was by experimenting with gradually higher and higher tire pressures while paying really close attention to how my car's traction changed at each stage.

Once you get your tire pressure high enough to notice the subtle feel of a tiny bit less traction simply let a pound or two or three of air out of the tires and you'll have found the sweet spot that works best for your tires/driving conditions.

For example, I got up to 52/50psi before things got sketchy and traction control light blinked on certain potholes in the road. So I backed down to what seems to me to be a sweet spot of 48/46, which is ideal for the roads I drive on.

The key benefit tp this type of experimenting is not just better MPG, but that you'll have improved your skills in sensing tire traction/performance, which is going to do a lot more to protect your child in the long than just obeying a warning label.

 

Firstly, I believe that the tire pressure Toyota recommends will give you the most comfortable ride.

Personally I never recommend you go over the max load rating on the tires themselves, which varies, 44 to 51 PSI are common, but not the only possible max load ratings.

I never recommend the ideal gas mileage tire pressure, as it is often unsafe. I try not to be unsafe, or advise some one else to be unsafe.

What I recommend is the tire pressure that makes your tires wear evenly, not bald in the middle first, not bald on the edges first. I believe that a tire that is evenly wearing on the ground, is also gripping the most. The best handling tire should be the safest tire. (it is true that it will give better MPG than the most comfortable tire, but that is not what I tune for)

 

If she is the only driver this is a sound approach. I would hate to hand the keys of a highly inflated Prius to an unsuspecting guest driver and wonder why they crashed two blocks down the street.

 

There is no ideal pressure, it is a compromise between competing goals. Higher pressure produces better mpg without limit (though the tires themselves will have a structural limit), but with rapidly diminishing returns. But higher pressure also reduces comfort, producing a harsher ride. Tread life, cornering crispness, wet traction, snow traction, and hydroplanning risk also change with pressure, though there is significant disagreement here how each changes.

So 'ideal' also depends on the road conditions and driver preferences.

The 40 psi limit for bead seating refers to the mounting process at the tire shop. After the tires are mounted on the rims, then you can consider the other limit, typically 44 or 51 psi depending on tire model and the manufacturer's labeling practices.

Some hypermilers go above the sidewall limit and report few or no problems other than harsh ride. But for liability reasons, most posters suggest not exceeding the sidewall limit. So don't add any more pressure than you already have.

 

NASA and other aviation sources indicate that higher pressure reduces hydroplaning risk. This matches my experience with a previous car that was labeled for 26 psi. It hydroplaned easier than anything else I'd ever driven, but pushing the pressure up to 32 made it behavior 'normally'. Living in the rainy Pacific Northwet, this is a very important factor to me.

NASA's empirical formula was that the hydroplaning onset speed increased proportionate to the square root of tire pressure. I.e. increasing tire pressure by 20% increased the speed where hydroplaning starts by ~10%. They tested aircraft tires to far higher pressures than cars use.

Traction on wet roads, without hydroplaning, is more controversial. I'm in the group experiencing no wet traction problems from higher pressure, but there are numerous people here in the opposing group. You will have to make your own choice.

 

I always agreed with this but crappy integrity tires my car came with were terrifying at 50psi at freeway speed in heavy rain. Was noisy as all get out and genuine loss of steering for very brief moments. So I'd suggest that it has much to do with tire type. The problem went away when I lowered psi.

 

Was hoping you weren't running Integrities. Yokohama is a very good tire. I would inflate those to 40 front & 38 rear. Anything higher and it will beat the heck out of the car on the roads you drive up there but still give you a very good mileage gain. I drive 42-40 in constant rain and never see the traction light. 30 psi and your mileage will really suffer.
And I bet your seeing the tracton light over potholes which will light no matter what tire pressure you run.

 

Actually, the opposite is true. Higher inflation makes hydroplaning less likely.

 

I think you are correct about the hydroplaning, but I believe the comment had to do with wet-weather stopping. I think Fuzzy1 brought out the NASA study supporting what you've noted about hydroplaning, but not everyone will agree that stopping distance isn't affected by tire pressure -- wet or dry.

I'll also add my two cents about the tire pressure discussions. You have to remember the tire, the conditions and the driver all have to go into the "calculus" used to determine acceptable or appropriate tire pressure. I changed tire pressures when I changed tires. It tood me some time to figure out a good pressure for my driving habits. I usually have the highest pressure when I am on long distance drives. I also will change tire pressure seasonally, going a little higher in the drier and colder times and lower in the wetter times.

 

Yes it's true. As long as you don't go too far, an increase in tire pressure will usually improve handling rather than impair it. I'm not recommending 50 psi, but certainly at 40 psi there should be nothing that is worse (performance wise) other than a slightly harsher ride.

 

I won't recommend 50lbs either . . . but I know my pressures are up there, & I found out how great some aspects of higher pressures are, after taking advice from some of our own moderators.


SGH-I717R ? 2

 

Just my two cents worth. The higher the air pressure, the better the mileage is, as well as the rougher the ride.

Concerning hydroplaning, the faster you drive, the better the chance to hydroplane. That is a given fact. So, with that in mind, consider slowing down some. Raising your air pressure to lessen your chance of hydroplaning may sound like a good idea, however if you are concerned about safety your best bet is simply to slow down a little bit.

I have seen many big SUV's fly by me during a heavy thunderstorm only to later see one or more of those SUV's lying upside down or on it's side in the ditch further down the road. Bottom line, he or she was just driving too fast and lost control.

Drive safely and arrive alive.