Calmchaos

Heya
Ive notice that the outsides of my front tires are wearing a little fast, so I ran out with my pressure gauge today, and they both read 30-31psi. The gas cap says that the pressure should be 29front, 36back. My back tires are reading 33.
My tires are 205/55/16 on all 4 rims, and the tag says 215/60/15.
Would the pressure have to be different due to the different size? Or should I just do the 29F and 36R like the tag recommends?

GlenL

Is that both outside on both tires? That indicates underinflation. Outsides like left side on left tire and right side on right tire indicates too much toe-in.

Don't worry about the stated pressures. That for the OEM tires twenty years ago. Find the pressure that rides and handles right along with the right wear pattern.

Calmchaos

The outside and inside on both tires.... sorry.

Bill.e1

I had an 84 Corvette a few years back with 255-50 16's. The wear pattern was opposite tradition- overinflation would wear the outside because the belt overconstrained the middle (something I experience every morning getting dressed but that's a different subject).

The inside/outside wear model applies to unbelted bias ply tires. It would probably apply to unbelted radials but I don't think there is such a beast. It probably still is somewhat applicable to higher profile designs but ultra-low profile performance tires- probably not.

Set the pressures so the tires feel good to you. If you use the car as intended, you'll kill the tires long before you see any pressure effects anyway ;-)

BTW- if you get nibbling or resonance through your steering, try dropping the front pressures a bit. A softer tire can dampen some types of oscillation.

Have fun,
Bill

Matt H

Wow, lots of misinformation here.

Wear on both shoulders indicated underinflation, not overinflation (which is indicated by wear right down the center of the tire).

If you were using the proper tire size then the factory sticker is the inflation pressure you should use. If you begin to deviate from that inflation pressure the tires might go below the minimum load carrying capacity for the vehicle.

For example:
Stock Tire 215/60VR15
Load carrying capacity at 31 psi = 1301 lbs
Load carrying capacity at 36 psi = 1477 lbs

ASSUMING you are using standard load Euro-metric tires with the highest load index
Your Tire 205/55R16
Load carrying capacity at 30 psi = 1179 lbs
Load carrying capacity at 33 psi = 1268 lbs

If you are using tires with a lower LI it will drop even more. Just drop it until it feels right is not the proper way to handle tire pressures.
__________________
Best Car Insurance | Auto Protection Today | FREE Trade-In Quote

951North

Question for Matt, while you were on the subject of Load Indices is that static load index or is there a dynamic allowance built in there also, say for instance our cars weigh 3000 lbs and under braking or cornering with weight transfer maybye we could see 1300 pounds at a front corner, albeit only for a short duration. Would you have to choose tires that would meet these dynamic loads?

jason

Scott at Team Harco

Interesting question. Unfortunately, there are many variables when it comes to vehicles, their use and the habits of the drivers. Tire load ratings are meant for static (or normal) loading. The 'official' rule on this is that the maximum normal load on a given tire shall not exceed 88% of the maximum load rating (usually embossed on the tire sidewall). This is also at the maximum inflation pressure of the tire (also embossed on the tire, in most cases).

Beginning June 1, 2007 this becomes 94% of that maximum load with tire inflated to the vehicle manufacturer's recommended cold tire pressure.

So to indirectly answer your question - there is some margin built-in to account for dynamic loading, though it is not specifically considered in those terms. As a general - common sense - rule, you should have tires with load ratings well above the maximum (static) loading possible for your vehicle. The tires should be properly inflated - never underinflated - and an appropriate speed rating should be considered. This latter attribute is probably as important as any when it comes to dynamic load capacity of a given tire. Don't skimp when it comes to the speed rating of your tires.

951North

Interesting, how many people actually run those types of pressures though? what 44-50 PSI Maximums... strange they would be increasing the percents (narrowing the margin between Normal to dynamic).

jenket944

And to throw another wrench into the discussion...
If you go +1 on the wheel size (from 15" to 16") you typically need to keep a little more pressure in the tire to compensate for the shorter sidewall. I had this issue when I put 968 wheels on my NA. Had to add about 2 psi over the OEM ratings to get the wear pattern to even out.

SchnellerUmsetzer

The wear could also suggest bad ball joints!

Scott at Team Harco

The logic behind the federal rating beginning in 2007 is to make it more realistic to 'real-world' situations. The reason the percentage goes up to 94% (when compared against 88%) is that the recommended cold tire pressure is less than the tires rated maximum pressure (and some of the load carrying capacity is used up at this lower pressure). You are correct - this is closer to the maximum load carrying capacity of the tire. You are also correct that few if any people run their tires at the max pressure. Nor should they.

The proper pressure tends to be at or near the manufacturer's recommended cold tire pressure. The upcoming rating is more realistic in that it is taken at this recommended - real world - pressure. To drive your vehicle with your tires inflated to the maximum pressure is sure to make your dentist and chiropractor happy.

On the surface, going to 94% seems like the margin for safety is being reduced. Actually it is not. It is just a different and more realistic way of looking at the same thing. Also, keep in mind this is the minimum standard as defined by FMVSS. In reality, most quality tires that you and I are likely to buy, far exceed this minimum.

Tom

For what its worth the recommended pressures for my 944S with 16" wheels were exactly the same as for my 84 944 with 15" wheels so it would seem the size of the wheel tire (205/55 225/50x16, 215/60x15) doesn't matter vs Factory recommendations. Individually our goals may be quite different (factory were for fuel economy, safe amount of understeer) so we might deviata a lot and use different tire pressures as well.

Matt H

By a HUGE margin at that.

Tom, did you work for the Porsche factory? If not dont make claims like fuel economy, understeer, etc. At Toyota fuel economy is near the top of the list, at Porsche it doesnt even show up. Performance is at the top of their list. Tires are designed for high performance cars with performance in mind. Safe amounts of understeer are built into things like the alignment settings and suspension components.
__________________
Best Car Insurance | Auto Protection Today | FREE Trade-In Quote