Tire Pressure for a layman

From what I understand, the 40 psi limit has to do with the bead when installing the tire on the rim.

The max load of 44 psi is the max tire pressure after the tire has been installed. A lot of members run their tire pressure all of the way up to the 44 psi limit.

I am currently running mine at 42 psi front and 40 psi rear.It feels fine to me as I have been used to running about 36/34 with my other cars.

 

As Jdcollins5 mentions, most of us run our tire at near the maximum listed on the sidewall (44 psi in your case) with good results. I think, based on what I have read and my own observations, higher pressures definitely increase fuel mileage, decreases the tendency to hydroplane, provide a firmer ride (which I like), and maybe do not have quite as much traction on dry pavement. Surprisingly tire wear seems to be better at higher pressures, I have noticed there is still a tendency to wear the outside edges a little more even at max inflation pressures.

 

Another interesting note: My Prius was delivered by the dealer with 39 front, 37 rear, so I think they are in on the overinflation trick. I was getting mid-50's, but the road noise was bad, so I deflated them to recommended, and it is a different car - with about 50 mpg. I bumped them to 37/35, and the trade off is good: decent road noise levels and 53 mpg or so.

 

The statement on the tire sidewall should read "max load xxx lbs at xx PSI". It indicates the maximum load the tire can carry (on the right rim). Sometimes it will tell you what the right rim size is.

The "do not inflate over xx lbs" indicates the max pressure before damage can be caused easily by road irregularities.

Both numbers have large safety margins built in.

The "recommended tire pressure" on the door placard is set by watching for even tread pressure distribution, at speed, over a glass plate in the roadway (with a high speed camera below). Sometimes I think it's a guess, however.

If you -under-inflate- the tires the edges of the tread will wear faster than the centre and the tire may overheat at higher speeds. You will also get poorer traction in snow and on ice, but better traction in deep sand.

If you -over-inflate- the tires the centre of the tread will wear faster than the edges. The tires won't overheat. You will get better traction in snow and water (the tread pattern will "clear" better), and even on dry pavement, until the over-inflation actually picks up the sides of the tread. I doubt you could reach that pressure without a blowout, however.

Do keep in mind, modern tires have a WIDE range of "correct inflation" pressure, due to the radial construction and modern rubbers used. This is why members of this forum don't usually notice the centre wearing faster than the sides of the tread. The tire can keep a more even tread pressure on the road over a wide air pressure range.

 

I wouldn't say most of us over inflate. I set mine to the factory recommended pressure.

 

let's decrease traction over a few miles extra per tank, that's just great logic.

 

Here in the Pacific Northwet, improved wet traction and improved miles per tank occur in the same direction.

 

+1

I tried adding 5 PSI for a while, and it made the ride noticeably harsher (and therefore more interior noises), and over a few months added a mile or 2 to MPG. Wasn't worth the trade-off to me.

 

Same here!

 

Overinflating tires is wonderful . . . For Tire Sellers.

Watch the tires wear. If they wear down the center the tire is Over Inflated. If they wear on the edges, underinflated. Tires also grip the highway, especially when the road surface is wet, far better when the tires are at the factory inflation pressure. Overinflation makes for easier Hydroplaning in the rain. Engineers compute the inflation pressures for the manufacturer, which is why the sticker on the door post showing that pressure can vary from one car to another. Since most people know nothing about automotive engineering, it is far safer and more sensible not to listen to the clucking on these websites when it advocates against the factory instructions.

 

By this rule, the placard pressures on all cars I've owned (pre-Prius, which hasn't been here long enough to know) have been too low.

Sources?

The aviation industry appears to disagree. See equation 1 and figure 4 here; and an equation described here. See also here and here and NASA.

Excessive susceptibility to hydroplaning was why I abandoned the 26 psi label recommendation on a previous car. A 25% pressure boost significantly improved its behavior here in the rainy Pacific Northwet.

 

I watch mine pretty closely and measure the tread depth every 5K miles. After 20K miles they are fairly even but the inner and outer edges are wearing faster than the center and I use 46 to 48 PSI (51 PSI max tires).

The old rule of thumb about causing excessive tire wear in the center by using higher pressure dates back to the days of bias ply tires. It does not apply to modern, good quality, radial ply tires. Tires used on the Prius have enough stiffening belts to keep them level across the tread at well beyond the max inflation pressure.

 

Hey you can do whatever you want with your Prius, recommended pressure or over inflate, I just wanted to clarify that its not standard procedure to over inflate them. I applaud the hyper-milers that want to squeeze milage from every drop of gas. I'm just happy with the normal pressure and its more compliant ride.

 

My month old 2010 Prius has the Goodyear Assurance tires. I noticed no difference in ride going from the delivered 35 psi to this group's recommended pressure around 40. I did notice a difference in mpg.

Lower friction, less wear and tear on your car and also on your tires. Keep in mind it isn't in Toyota's best interest to have you drive your car for a long time. They don't care about a little extra wear and tear. Also, they don't care if your tires wear out a little early from the extra friction.

My fuelly page:
GBC Texas Prius (Toyota Prius) | Fuelly

 

You are buying crap tires for the sake of low friction, overblowing a tire to save a buck or two. I don't know about you, but I care about the traction way more.

 

Heck ya Fuzzy! I agree! I inflate to the max psi and it is very effective! I never drop below 51mpg!

 

Good references all in one place. Bottom line, it is indisputable that higher pressure leads to significantly less hydroplaning for a given tire.

On the other hand, higher pressure leads to a smaller contact patch so it will increase dry braking distance, how significant this effect is I do not know. Higher pressure will also lead to stiffer sidewalls leading to more responsive steering.

Overall, I have chosen to run my Prius V at 38/37 rather than the recommended 33/32.

Rumple

 

The best way to test this is obviously change the pressures and hit the brakes, but I want to mention that when a car is braking hard, the weight shifts to the front, so having a higher pressure than what is on the doorjam may actually be more appropriate for hard braking.