Tire inflating

[phantom power]

This is a car question, but relates to saving fuel. Plus I know some of you know cars.

A few months ago we got a Volvo, which came with Pirelli tires rated for 50psi. I wanted to inflate them to at least 45psi, but the car's manual says the tires shouldn't be inflated more than 38psi. I suspect that the manual's limit applies to the factory standard tires, which I doubt are Pirellis.

Is there any reason I should stick to the manual's limit, or can I simply go by the psi rating of the tire itself?

[tubbacheez]

1. I think the tire maker's advice trumps the car maker's advice on this.

I'm not a car expert, but my mechanical engineering background says that - provided Volvo has certified these tires for use on your model of car - then you should follow Pirelli's advice on their proper inflation.

[htuttle]

2. I'd go with 45 and see how it handles

Seven psi above the vehicle's rating isn't that much, and the tires should be able to handle it if they are rated at 50psi.

BTW, I've never heard of automobile tires with such a high pressure. 50 psi is something I'd normally associate with bicyle tires, not car tires.

The one thing to worry about would be whether the rims themselves could handle the extra pressure, but they *probably* can.

Just stand way back behind something sturdy when you fill them.

:evilgrin: (kidding...but only a little bit...)

[phantom power]

4. I was surprised, too. But it's right there, on the sidwall.

I was talking with my brotherinlaw, and he said newer tires are trending toward higher performance. Bigger rims, narrower side-walls, higher pressure.

Sounds like a theory. I just want better gas mileage and longer tire wear, but I also don't want them to explode on the highway, or in my face.

:hide:

[stevedeshazer]

3. It's on the sidewall of the tire (max pressure)

Trust both.

As a practical matter, you'd be good to go with a compromise between the maximum pressure recommended by Pirelli and the Volvo dealer.

Pirelli wants maximum wear. Volvo wants maximum ride.

Splitting the difference is reasonable.

45 PSI is gonna give a stiff ride. It'll work, but you'll get a rather stiff ride.

Most vehicles recommend around 32 PSI for tire inflation.

[Logiola]

5. Use the Car rating

ignore what it says on the tire.. The rating on the tire is "MAXIMUM" it does not mean what you should put in it. THe ratings on the car (usually on the inside of the drivers side door, are what you should use for best gas milage and handling. (you can go one or two psi over if you please for a little firmer.

it all has to do with your cars weight and how it is meant to handle.

[happyslug]

6. Higher tire pressure better fuel economy

But lower tire pressure better traction, all other things equal. 45psi is high for a Car, trucks often have higher pressure for the same reasons bicycles have higher tire pressure, less roll resistance and thus better gas mileage (or in the case of a bicycle, easier pedaling).

I would hesitate going with anything higher than what your car manual is suggesting, for the simple reason your car was designed to operate THAT TIRE AT THAT PRESSURE. In some cars the tire pressure makes no difference, in others critical (for example the early Corvairs were very sensitive to tire pressure and the chief reason their were "unsafe", the same with the M151 series of Vietnam era Jeeps).

Volvos are a well built and designed car, if the tires are marked 50 PSI, but the car manual is saying 38, Volvo has to have some reason for the difference. It may be that the suspension of the Volvo depends on the lower inflation number to meet the the comfort level Volvo wanted, or it can mean that at high speeds and/or turnings the tire will gets so warm that it will "explode" and thus Volvo wants the lower figure to provide a sufficient safety "cushion".

Given the higher Gas prices in Europe (and Volvo is marketed for European Drivers as while as American Drivers) I lean to Volvo Just wanting to preserve some safety margin. Remember as a tire rotates as the car is driven, the air pressure in the Tire heats up and expands INCREASING THE TIRE PRESSURE. Volvo may believe that if you keep the tire at 38psi when the tire heats up the tire will NOT go over its maximum pressure and burst. Perelli may believe that the correct pressure is 50, so to get maximum fuel efficiency with its tires.

Given the above I would lean to stay with the 38psi, unless I learn for sure (and you never will) that the concern was comfort not safety.

[bemildred]

7. Agreed.

I try to run vehicle manufacturer spec plus 3 lbs or so, which is
say 35 lbs, if it says 32 lbs, in the case of 38 lbs would be about
41 lbs; and check it once a month or more. If you get too
far off the tire doesn't wear even etc. When there is an "extended
high-speed driving PSI" use that.

[phantom power]

8. Thanks for the advice everybody.

I've been inflating to 40psi, sounds like that's a pretty good place to keep them. Maybe I'll go on a rampage, and set them to 41.