confused about tire pressure

It does depend upon which tyre you've got. I'm going to assume that you have the OEM Goodyear Integrity tyres. In that case, I believe that they are rated up to 44 psi. That's the highest safe pressure that you should maintain. There are quite a few folks out there that keep their tyres at 42/40 (front/rear). I keep mine at 40/38 but only because that's what TonyPSchaefer does. Seriously, though, the ride will be somewhat rougher the more you inflate the tyres, but road conditions will effect how that translates into the comfort of the ride. If you're blessed with pretty smooth roads then you probably won't notice much. Folks have reported that increasing the pressure does seem to translate into increased wear on the tyres.

 

Isn't it supposed to increase fuel economy, too?

 

The whole point of "overinflating" tires is to reduce the size of the "footprint" a tire has on the pavement at a given moment. (Compare racing bike tires and mountain bike tires.)

I disagree with what Tripp reports others' saying. In 45 years of inflating tires on 36 cars above what the door jamb/glove box sticker says, I don't think I've ever incurred more treadwear than the tires should have gotten. True, at some point in too much overinflation, you'll get disproportionate wear toward the center of the tread (as opposed to what most people get, which is premature wear on the edges caused by underinflation). But there's a pretty wide latitude where you'll get even wear.

Just remember, at least once a month, to turn your front wheels as far as you can when you park to give you a clear view of the entire width of the tread close-up. Look especially at the shallower grooves near the edges. Does the wear look uneven?

ex-NYer

 

The manual says higher pressures are "unsafe" only because the chance of puncture is slightly greater. In actuality it is much more unsafe to run them underinflated. I run mine at 42/40, not only because of the increased economy but I like a firm ride. Also I find tires on my cars usually wear more on the outside than the middle tread because there are lots of curves and corners in my area, increasing the pressure helps slow this pattern.

 

Exactly! Well put! I'll add that the largest "danger" in overinflating your tires is that you reduce the footprint so much that you don't have good traction on the road. Any amount of inflation (within a reasonable range, of course) is going to be a trade off between traction and mileage. If you inflate your tires more, you will have a smaller "footprint" (or, should it be "tireprint"?) on the road, so you'll have better mileage and less traction/control. If you inflate your tires less, you'll have worse mileage and more traction/control. The 42/40 level has been choosen by people here as a "safe" compromise between safety and fuel economy. I should point out that there's no real evidence that this is a "safe" pressure, aside from the fact that we're all still here to post on this board

As someone else pointed out, more inflation will also lead to a (somewhat) rougher ride. If you're in NYC, all of those potholes may make you wish that you hadn't inflated your tires so much. . .

 

Pressure is good! Of course I'm coming from the world of EVs and high-pressure Low Rolling Resistance tires, so 50psi is the norm there.

Honestly though, in ALL cars I've owned, I air up to near the max of what it says on the sidewall. With increased pressure (to a point, mind you!), you get better handling, longer wear (they get less hot) and better mileage. I can't come up with a logical reason why a tire at 40psi would be more prone to puncture than a tire at 35psi. Yes, it is harder to puncture a deflated balloon, but in these cases, the tire is *almost* equally hard relative to something that would puncture it.

 

Modern tires bulge hardly at all with higher pressures, so uneven wear is not a problem. Anything up to the max on the sidewall is safe *for the tire*, but the car's suspension and handling were designed assuming the pressures given on the door pillar; using a different pressure will change the handling noticeably and perhaps the car's stability. I use 40/38 and am still pondering the value of slightly better MPGs versus pounding the chassis a little harder. It makes me drive a little slower and more careful of avoiding road defects.

 

Actually Increasing the tire pressure to 42/40 INCREASES the mileage you can expect from the tires. When i first got my 04 Prius I didn't raise the pressure and the tires wore out at just 25k. My Prius tech told me to go to 42/40 and the second set of Integrities had plenty of thread left when I put another 20k on that car. Even at the increased pressure the outside of the tires still wear first.

 

In summer with my Michelin Harmony tires, if I run them at 44/42 instead of the recommended pressure, there is no difference in city fuel economy. On the highway, I will usually get slightly better fuel economy, such as 4.4 l/100km instead of 4.5 or 4.6.

A difference of about 3 MPG Imperial gallon.

Given the condition of Winnipeg city streets, there is *no* way I'd run the tires higher than 35/33. I find my Yokohama Ice Guard winter tires are rated a higher load at only 35 psi, and they have a nice soft ride. Makes those potholes and frost heaves much easier to deal with.

 

An interesting topic that comes up frequently, I'll throw my two cents worth, which is only my subjective opinion:

I think the notion that the center will wear faster if the tires have more pressure in them is an idea that is left over from the days when all/most cars had bias ply tires. With bias ply tires the whole tire will grow when you put air in it and the center of the tread will grow more than the outside. With radial ply tires there is enough strength in the area below the tread, where the belt is, that the tread stays fairly flat without reguard to pressure. People have reported here that even with over 40 psi in the tires the outside edges seem to wear out first. I haven't had the Prius long enough to observe this but I have seen exactly the same thing on my previous car with 42 psi in it's tires. That is, the edges wore out first with both the Michelin MXV4s and the Bridgestone Potenzas I used on it.

There is a definate and measureable increase in mileage when you increase the tire pressures. This is not just true in the Prius, but in other cars as well. For example in my Honda Accord increasing the pressure from 35 to 42 psi in all four tires increased highway mileage by a consistant 5% to 10%, I see about the same percentage in the Prius. This is due to less tire flex in the sidewalls and a slightly smaller contact patch, thus lower rolling resistance. There should be a small increase in city mileage as well but I don't know how much.

I can't verify either of these things from experience but believe them to be true. 1. There should be a slight penalty in handling with higher pressures but on street cars it is most likely not detectable. The term handling as used here means ultimate cornering ability of the vehicle not what the car feels like to the driver. 2. It has been reported, here as well as on other forums, that resistance to hydroplaning actually improves with higher pressures because the tread tends to remain more open and drains faster.

When I increased the tire pressures in my Prius from the factory recomendation to 42 psi in all four tires I noticed only a slight difference in ride if any at all. Despite what I said in the previous paragraph I "felt" that the car handled better, I think that is because it would steer into the corners better because of less sidewall flex. As far as I could tell tracking, how well it remains in a straight line on the highway, was unaffected.

 

Somthing not noted here is that lower pressures also means more heat. As sidewalls flex they generate heat and instability in high speed turns. Remember Ford explorers and low tire pressures? How many of you out there in tire land have had blowouts or punctures often? I sure don't, but I have had lots of tires fail in their sidewalls. Heat is probably the single worst enemy in tires today with steel belts protecting the tred.
-Paul R. Haller-

 

Crap, I'm a tool. I left out a negative. That's what I get for not proof reading the bloody post. What I meant to say was that others have reported that increasing tyre pressure does NOT seem to increase the wear to any appreciable degree. Thanx for the catch, Jack.

 

I used to run my tires at 40/38, but with so many potholes my car begins to rattle madly. Therefore, im back to 35/33...smoother, but sacrificing 3-4mpg.

 

This is quite true... increasing the pressure will change all those things - for the better! It'll change the ride quality (improve it in my opinion), and it will typically increase the parameters of performance that you've mentioned here, and the tires will last longer. Everybody wins except the guys selling tires, and the guys who like a softer ride.

 

I increased my tire pressures to 42/40 on my 05 Prius and found that the actual increase in mileage was very small. Most of my driving is suburban with some interstate. I now run my tires at 38-36 and feel that I get a much better ride, and only a modest decrease in economy. The difference was less than 2mpg. I average right at 50 mpg at 38-36. When my new red 06 comes in next month, I plan on running it at 38-36 psi. Some of my friends with their Prius have had the same experience. Love the new red color! B)

 

The tire pressure sensors. Do they show current tire pressure or just let you know when they are low ? If so, what pressure does the warning happen ?