Proper tire inflation question

Factual Questions
1

My attempt to find out the recommended p.s.i. for my tires (which say only “max. pressure 44 lbs”) led me to a site which advised that tires are to be inflated to the pressure specified on the car manufacturer’s tag inside the door. Maybe this is a dumb question, but how does the manufacturer of the car know what the correct pressure for replacement tires is? The car manufacturer presumably knows nothing about the tires except their size, not their type or composition. How is the manufacturer supposed to know how much pressure they can handle? Assuming that the manufacturer’s tag has been effaced, and I don’t have access to the manual (the car is a 1998 Mitsubishi Galant which I bought used), then what do I do?

I’m embarrassed to be asking such elementary questions at my advanced age, but I feel that I have destroyed more than my share of tires in my life (usually through underinflation, though I did have one tire blow up on a hot summer day because I overinflated it and drove on it), and I’m ready to start practing good “tire hygiene” as it were. Thanks for all informed replies – no guessing, please.

2

The recommended tire pressure from the manufacturer will be at or below the normal pressure for the tires. Most passenger tires used to be 32psi but some cars reccomended running below that for either handling or comfort.

People used to reccomend that you ran the tires at the maximum pressure for milage reasons.

Tire stores no longer do this because of handling issues. I seem to recall one of them was with the Ford Explorer in the early 90’s that handled much worse at max pressure than at reccomended pressure.

Take all the above information with a grain of salt as I have not worked with tires since the early 90’s.

3

You are correct in that the door label pertains to the factory recommended tires. If you opt for different tires, that may change. Example-my 1985 E150 had stock tires with a maximum pressure of 32 PSI. I opted for replacement extended load tires of the same size with a maximum pressure of 41 PSI. I believe that one should always read the sidewall of the tire and follow that.

4

That and find a happy medium. Just because a tire is rated at a “maximum” inflation level doesn’t mean you should inflate to that. Most street tires are perfectly happy running from 32-38 psi.
Of course, your mileage may vary… :wink:

5

The pressure on the tire sidewall is a safety limit not to be exceeded (not a “normal” pressure), and is determined by the tire manufacturer. It is not intended to be, nor should it be used as, a recommended pressure for the tire (space-saver spares are an exception, but clearly marked to that effect). Recommended pressures vary with which vehicle the tire is used on, and are determined by vehicle manufacturers. Sorry, I don’t know how the engineers calculate that, but since car makers DO furnish these specs, and tire makers DON’T, one has to assume it’s a valid process.

Manufacturer’s specs are found in different places on different cars. These include any of the doors (rear-facing edge), any of the door jambs (front-facing edge), inside the glovebox door, inside the glovebox, and in the owner’s manual. Many (most?) vehicles have the specs in only one of these places. If you can’t find the info on the car or in the manual, it should be available from a dealership.

6

I would add inside the gas door lid to the locations to look for the inflation sticker.

Inflation pressure is a function of weight, weight distribution, and handling. It is not a function of tire brand.
The car maker knows what pressures work the best for each vehicle. There will be two pressures listed. A recommended, and an optional. The optional (higher) pressure will result in slightly better fuel mileage at the expense of ride quality. (I don’t notice it, but some people do)
FTR many tires today run at higher than 32 PSI. For example a new Volvo has a recommended pressure of 39 PSI and the tire pressure warning system alerts at 20% low, so the low tire alert comes on at 31.2 PSI. Assuming the gas station has a dead nuts accurate tire pressure gauge (good luck with that!) and you set your tires to 32PSI in the middle of a warm afternoon, the next AM you will have a low tire warning. (I hate it when that happens) :smack:
Tannim The problem with the Exploder was that they specified a lower than normal pressure (26PSI IIRC) for ride quality. Customers then didn’t check their tire pressures often enough, and as the tires bled air, (normal) the handling got squirrelly. At about 15PSI or so the vehicle had a bad habit of either swaping ends or turning over as it tired to swap ends.

7

Interesting. I run the 10 ply load range E Cooper tires on my E250 at 80 PSI and they wear like iron.

8

On some vehicle/tire combinations, the recommended pressure is the same as the tire’s maximum pressure. The thing to be aware of is, it’s the recommendation because the vehicle manufacturer said so, not because the tire sidewall said so.

9

The gas cap sticker says 55F/75R, but Coopers aren’t standard Ford issue sneakers, although LR E is standard for E250. I’ve run them at 80 front and back for years without problems. I can’t imagine only 55 in the front.

10

Thanks, guys.

11

I disagree. Different tire brands have entirely different characteristics. Some tires have softer sidewalls than others and may require more pressure in order to avoid excessive sidewall flexing during cornering.

The psi numbers given by the manufacturer are just a recommendation that should work adequately with most tire brands. Finding the optimal pressure for a given set of tires requires a lot of trial-and-error

12

One way to figure it out yourself is to air the tires up and drive through some water or powder you’ve shaken out on the ground then examine the tracks the tire leaves. You want the entire tread touching the ground perfectly flat. Both sides and the middle of the tire should all leave the same mark after going through the powder

13

Back in the day when you could by bias ply, bias belted, and radial ply tires, I would tend to have agreed with you. But now that everything is radial ply, the differences in construction is fairly minimal. Sure Danceswithcats cooper truck tires are going to have a way stiffer sidewall than the passenger car Michelins on my station wagon, but that is apples to kumquats. If we compare just tires for Dances’ truck or just tires for my wagon, the differences in sidewall stiffness is fairly minimal.
The two exceptions to this are weight saver spares, and run flat tires.

14

First of all Peugeot and Citroen specify along with the pressures tire brand and model: ImageShack - Best place for all of your image hosting and image sharing needs

Now a few things about tire construction I’ve learned from tire company seminars (I operate a tire shop) and from experience. There are two main schools of tire construction.

The first is using a hard, long-lasting tread compound combined with soft sidewalls/carcass. Although hard compounds do not have good traction, the soft carcass permits more tire to come into contact with the road. Most Michelin tires are designed like this.

The other school is using a soft tread compound combined with stiff sidewalls/carcass. Stiff sidewalls have the advantage of making car control more precise, especially at transient situations (S-curves in quick succession). The advantage of these tires is that they have a sportier feel but wear faster. Pirelli PZero Nero is designed like this, other Pirelli tires not so much.

Tires for low-end cars that are marketed as “fuel-saving” have both stiff sidewalls (less rolling resistance) and hard tread compounds (less rolling resistance and long lasting). Needless to say, those tires have terrible traction and unless mileage is the main concern, they should be avoided.

And a real life example: My car comes equipped with Michelin Exalto 2, size 205/50/17. I changed them to Pirelli PZero Nero size 215/45/17 (sidewall height roughly the same with original, about 1cm wider tread) and took the car for a test drive on my favorite mountain pass. I noticed that the sidewall flexed so that part of it actually touched the road surface under heavy cornering. I never had this problem with the Michelins, even on the track.

Here’s a pic. Notice the black line: ImageShack - Best place for all of your image hosting and image sharing needs

I had to inflate the tire to ridiculously high pressures to avoid that flexing, but then the traction was worsened. Maybe the tire is deliberately designed to do this and that would also explain the presence of the “sharkfin” grooves on the sidewall. I doubt it was because the tire was wider, because I have also used Goodyear F1s size 225 and they didn’t exhibit this behavior. Too bad I didn’t keep these tires long enough to test on the track.

Later I found on a specialized forum that while most tire sidewalls flex uniformly, Michelin tires have the weakest part of the sidewall near the bead, while the rest is relatively stiff.
Of course all this is of little concern to the average driver who only wants to travel from point A to point B. Simply stick to the factory recommended pressure and that’s it. But, yes, different tires have entirely different construction and require different pressures. To get the most out of them you have to find the optimal pressure by trial-and- error.