In a few days I will fire up the school bus/moving van and head south. The bus has been sitting for two years (I have started it up and run it periodically) but in that time I am sure that the tire pressures have dropped.

The tire are bias ply (i.e., not radials) truck tires, and I have no idea what the correct pressure should be. I think it's somewhere in the 80-100 psi range, but don't know for certain. I have a mobile service truck coming by the storage lot this afternoon to change the oil, check the fluids, get the bus ready to roll, but I'd like to know what tire pressure to tell him.

If you had an old 44 passenger school bus, what tire pressure would you use?

tanstaafl.

Isn't it printed on a plate or sticker attached to the door jamb?

You could always take a pressure gauge around to a school (bus) parking lot and check first to see what kinds of tires they have and then take a sampling of readings from 10 or so tires. Go with the average.

75 psi is what I've seen on bus tires. But I'm no expert. I would think the service people should know. But like you, I like to double check this sort of stuff too.

Usually truck tires don't lose air pressure as bad as car tires. Thicker rubber I guess.

The guys on http://www.skoolie.net/forum/ might be able to tell you.

The pressure will be printed on the tires. Don't bother looking for a data plate, it may be wrong. My Volvo plate says 32psi, but the tires I'm running on it are 44psi. Of course, 18 years ago when the car was new, 32 was the norm.

I think the tires they use on school buses are 60psi. Definitely read the tire, it will be on there somewhere.

AHA, now this opens up a discussion about the appropriate tire pressure for passenger car tires. Which number is correct? The number on the tire or the number on the door jamb?

I say that the number on the tire is a Max Pressure number and isn't useful for your normal running pressure. On the other hand, how can the number on the door jamb be right when you buy a different brand of tire?

Here's what I've experienced, though:

- If I fill it up to the number on the side of the tire, that's TOO MUCH. The tires wear out more quickly, with the wear pattern being in a line down the center of the tread, and the edges of the tread not wearing hardly at all. This tells me that the tires are "bowed outward" at the center and thus are overinflated.

- If I fill it up to what's on the door jamb, that's TOO LITTLE. The tires seem like they have mushy squish on the sidewalls where they meet the road, and the wear pattern is that the edges of the tread wears out more quickly than the center of the tread. This tells me that the tires are "bowed inward" at the center of the tread and are thus underinflated.

So I go with a number that's somwhere above the door jamb number. I think my door jamb number is 30, and I try to hit 33-35. Higher if I'm traveling with a heavy luggage load.

I'm sure CarTalk has answered this question a million times by now, but I don't know what their answer is.

Funny thing about the tire pressure marked on the sidewalls.... Even when my (previous) car had stock tires, the numbers never matched between tire and data plate.

isn't the information on the tire MAX PSI? Which isn't necessarily what you want to inflate them to. The markings on the data plate are the recommendations for the application, based on the weights of the vehicle and its recommended loads.

That's one of the reasons why the numbers didn't match in my case. The car recommended different pressures for rear versus front tires. All four tires were the same brand/model and all had identical markings - all higher than what was printed on he data plate by about 25%. Tire says "44 psi" and car recommends 33.

There's not really any discussion about it. The "correct" number is the one on the car. Most tires clearly say "MAX PRESSURE" on the sidewall.

I put "correct" in scare quotes because there's never a single correct pressure. It varies based on a variety of variables, from the car's cargo weight, to temperature, to speed driven.

Inflating your tires to the maximum pressure will make the car run really smooth for a while, as you'd be running on about four square inches of patch. But it's not safe to have that little contact with the road, and it will wear your tires unevenly. Also, if you're pushing the tire's max rating, it's awfully easy to go over, and then you could have worse problems. Underinflating is almost as bad.

But the number marked on the car is far more accurate than the number marked on the tire, since the number marked on the tire was never intended to be a suggested inflation pressure. This is obvious when you consider that a large portion of the criteria to determine air pressure is weight, and cars with vastly different curb weights use the same size tires.

I'd be remiss if I didn't specifically point out that this is dangerous.

What happens if you change tire sizes from what the car shipped with? Is there some way to calculate a good "recommended" PSI? I went from 235/50-18 to 255/45-18 on my car, and stuck to the numbers on the door plate.

This is especially true for older vehicles, where tire technology has surpassed what was available at the time of manufacture.

My brother ruined six tires on his motorhome following the recommendations of the data plate. The tires demanded more pressure- he ran them under-inflated and they shed their tread. No fun.

Doug- I'd say the mobile service man will have a good idea of what to do, but in general I'd start with the 'max pressure' as stated on the tire itself, and then knock off a few pounds. For example, if the tire max is 80psi, you'll want to not go under 60psi or so. You might start at 75psi and drop down as you drive, based on how you like the 'feel' of it. Remember that tire pressure should be checked (and compared) when the tire is cold, so you have a standard to base your data on.

Running the tires at the maximum allowable pressure will give you a small contact patch (as already mentioned). This is great for fuel mileage, but horrible for safe control and ride comfort.

Good luck on the move!!
I'm out of town this weekend, but if you still need some help next week be sure to call me.

In my racing experience, we'd start high and lower the pressure until the cornering wear on the side of the tire started to get around 1/4 inch. This way we kept the tire from 'folding' (underinflation) and also maximized the contact patch.

It's really a trial and error thing.
You have to base it on your driving style, the conditions you're in, and the tire itself.

I encourage everyone to pay more attention to their tire pressures. A simple change of 5psi could be as important as a $500 tune-up wrt fuel mileage.

LOL!

Fuel mileage? Try 6--7 MPG. Ride comfort? A Conestoga wagon would be luxurious by comparison.



tanstaafl.

That's what the local tire shop said -- go Max Pressure and subract 10 pounds. So that's what I'll do.

tanstaafl.

LOL yes- and if you're under-inflated, you'll drop that even further.

It may be that the pressure listed on the car is wrong for one reason or another, but the pressure on the tire is never right.

If you really think that the listed pressure is wrong, you can always figure it out based on the shape of the tread. Find a smooth surface you can park on; smooth concrete would probably be ideal. Then get a feeler gauge. Overinflate your tires so that you can get the feeler gauge between the tire and the concrete on the side of the tire. Release the pressure until the feeler gauge no longer fits. You'll probably need to move the car a little bit after releasing the pressure to release friction between the tire and the ground. Record that pressure. Now release more pressure until the feeler gauge can slip between the tire and the concrete in the middle of the tread. (You might need to tape the feeler gauge to a stick to get it that far underneath the tire.) Mark that pressure. Now you have two pressures that you know to be too much and not enough. So set the pressure between them. Hopefully they will be close enough that you can draw some sort of conclusion.

I agree with this. The pressure listed on the tire is the maximum that the tire has been designed to withstand- not the optimal pressure (which is dictated by many variables).

As for your method of finding that 'optimal' pressure, I don't think the contact patch acts in as simple a fashion as you suggest. Under-inflation caves the contact patch, yes, but in all dimensions- the cave you see in a 2D cut-away is not accessible from the front or rear of the tire.

Have you performed this method?
I may just be confused by what you described.

Hm, yeah. You've got a point. Well, the sides of the tires should work to let you know when it's overinflated.