Tire Inflation

Factual Questions
1

Obama supporter that I am, I got a tire gauge, and searching around, found that the 15" tires on my 2005 Scion xB box on wheels are supposed to be inflated to 29 PSI. My mother’s itty-bitty little 1997 Kia something or other 13 or 14" wheels are also supposed to be 29 PSI.

But there’s some leeway here, isn’t there? Lower inflation means smoother ride, slightly better handling, slightly longer tire life. Higher inflation means better gas mileage. Are there other implications? How high should I go? Mom and I both do pretty strictly city driving, and not much of that - probably 2-3000 miles per year at most. I go on highways sometimes, but Mom never does. My Scion was designed for maximum interior room, rather than handling, but it does have power steering and some kind of special traction, as well as ABS. Mom’s car is just a little sub-compact.

Thank you in advance for your knowledge and advice.

2

How did you determine the correct tire pressure? The maximum tire pressure should be printed on the door pillar on the driver’s side. Most passenger cars will be in the 32 to 35 psi range. Make sure you check the tire pressure when cold, the tire, not you.

3

I got the correct inflation from both the car door and the manual, just as you suggested.

ETA: The tire was as cold as it gets in a comparatively cool day in a garage in August in New Jersey.

4

A few years ago, under inflation was associated with SUV/pick up tires coming apart on the highway and causing accidents. There’s a thought… Granted, the tires were defective, but under inflation apparently increased the risk.

Follow the instructions regarding your tires in the manual that came with your car. Frequently, there’s a range of values given (perhaps 28-30 psi, as an example) so if you’re really concerned about mileage you might opt for the upper value.

I’ve long checked my tire pressure monthly and adjusted as appropriate. I have not noted any handling problems and I consistently get good mileage. I would make the same recommendation to anyone else.

5

This page from the Car Bibles might help you make some sense of it all.

6

I wouldn’t list longer tire life as something you can expect with underinflation - it would more likely shorten tire life. I would recommend keeping the inflation between the specified value and 10% over that. The type of vehicle and type of driving aren’t significant issues here - the engineers who designed the car have figured out the optimum inflation pressure for the tires they chose to put on the car. The one point of variation I’ve seen - in the manufacturer’s specs - is a second, higher, inflation figure for the rear tires when carrying heavier loads. The thing to do is go with the manufacturer’s specs and check the tires often enough to keep them there.

Helfpful info here and here.

7

Just to clarify, “cold” refers to a tire that hasn’t been driven recently. Driving it from your house to a nearby gas station ought to leave the tire still relatively cold. Even if it’s quite hot out, and you haven’t driven, your tires would still be considered cold; they get much hotter than ambient when driven, especially at high speeds.

8

Low inflation means lower tire life; it will make the edges of the tread wear more quickly.

You can go as high as the tire manufacturer’s recommended pressure- it will be printed on the tire itself. This will usually be a bit higher than the (car) manufacturer’s recommended pressure, which is adjusted a bit to optimize the vehicle’s ride.

If the manufacturer’s RTI is X psi and the tire manufacturer’s pressure is X+10 psi (it probably won’t be that great a difference, though), I’ll go as high as X+3 psi. This isn’t a scientific standard or anything, though; it’s based on personal experience. YMMV significantly, depending mostly on whether you’ve got a big car on little wheels (ie. a minivan) or a little car on big wheels (ie. a sports car).

If you do go above GT’s recommended 10% over, your tire pressure monitoring system (if you have one) will probably come on, just FYI.

ETA: low pressure doesn’t really improve handling, either. It will give you a little more grip, but it will also increase body roll and reduce steering response. Also, in wet conditions, it will dramatically increase your risk of hydroplaning.

9

A little clarification - the pressure on the tire sidewall is not a recommendation per se, it’s a maximum which is not to be exceeded.

10

Thank you all very much. Sounds like I’d better just target the 29 PSI, or possibly a teensy bit above, for both my mom and me.

11

>Obama supporter that I am, I got a tire gauge, and searching around, found that the 15" tires on my 2005 Scion xB box on wheels are supposed to be inflated to 29 PSI. My mother’s itty-bitty little 1997 Kia something or other 13 or 14" wheels are also supposed to be 29 PSI.

Wow… amazing! Same here! Except my xB is an '06, and I bought a compressor, too (hardly any gas stations or garages around here has air available to the public). I, too, was surprised the recommendation printed on the label in the door frame was 29 psi, with no leeway. The manual said no different (I think it only referenced the label, though I was doing this for a few vehicles and don’t remember the Scion for sure).

The mechanics I have used for years had suggested they inflate the tires a little higher - I think 3 to 5 psi higher - on our older cars (the current cars are under warranty so I won’t see much of the mechanics for a while). They explained that there is a compromise, with lower pressure offering a quieter and smoother ride and higher pressure offering better mileage and tire life. I kind of think the tire life might be best with a small surplus of pressure, whereas inflating them to 100 psi (like in Fear and Loathing in Las Vegas) would probably give the best gas mileage, not counting the loss of a full tank in the fiery crash that would happen sooner or later.

Also, when we spend a week at the beach every summer, we drive the Jeep (or whatever) out to the water’s edge, and following the National Park Service’s recommendation, we lower the tire pressure fro 35 to 15. That’s right, 15 psi. This year I started out with 20 psi because our current Jeep has such poor clearance we had minor damage on the undercarriage last summer, and it did ride higher, but we got stuck. I lowered the pressure to 17 and we didn’t get stuck again. I have to wonder if I’m splitting hairs and imagining the difference. In any case, we drive around town, and even make the 15 mile trek to the far end of the island at 45 to 50 mph. This goes on for a week, and I am pretty sure all the other 4WD tourists we see are doing the same thing, because there’s only a line at the gas station air hose on weekends when most people are leaving. FWIW, and YMMV. Actually, YMMV all over the place.

>The type of vehicle and type of driving aren’t significant issues here - the engineers who designed the car have figured out the optimum inflation pressure for the tires they chose to put on the car.

I don’t think this is right, or at least think it’s too simple, if you’ll pardon the suggestion. Probably, using the manufacturer’s recommendation reflects a nice compromise between all the different issues, and may well be safest, too. But “optimum” has to mean “according to some set of criteria with varying importance”, doesn’t it? And if a smooth ride and high gas mileage are critera whose relative importance varies from driver to driver, mightn’t the optimum vary too? Granted, using a pressure different from the recommendation does leave the driver sort of on their own. But, an excellent guide would be most welcome. I’ll look around, including at the cite above.

12

It’s a small world, isn’t it, Napier? I’m glad to hear that you (and your mechanics) think that if I mistakenly inflate my tires to, say, 32 PSI, they won’t explode! It’s a good quality in tires, not exploding; I like it.

13

Under inflating tires does not improve grip with one exception. If you are off road in soft dirt or sand, under inflation enlarges the footprint, and can help you from getting stuck. Since there is no risk of sinking into pavement, the same does not hold true for road tires. Actually going above the car maker’s recommendations can improve handling.
Under inflation will also increase wear on the other edges of the tires. Under inflation also increases tread squirm and heat build up in the tires. This was the crux of the Ford Explorer tire issue. To get a good ride, the engineers specified a low tire pressure. Customers were not checking their tires often enough and when on the highway heat build up in the tires caused a failure.
Over inflation can improve handling and increase wear in the center of the tire (If taken to extremes)
Often the owner’s manual or sticker on the car lists two pressures, one for normal conditions and one for a full load. the full load pressures will be higher. You can safely run the higher pressures regardless of the load of the car. However if you are running fully loaded, you should for sure run the higher pressures.
If your vehicle only lists one pressure like my daughter’s truck you can exceed the listed pressure as long as you stay under the max pressure on the sidewall of the tire. On her truck the pressures listed are 35PSI front and rear. The tire lists 44 PSI as the max pressure. When I set these I will use 38-40PSI.
There are a couple of reason for this.

  1. My tire gauge might not be accurate.
  2. The higher the pressure, the longer I can go between having to fill them. :slight_smile:

One thing. Tire pressure are set cold, IOW before you have driven more than a mile or so. **Under no circumstances should you bleed air from a hot tire. **

14

Gary T: The type of vehicle and type of driving aren’t significant issues here - the engineers who designed the car have figured out the optimum inflation pressure for the tires they chose to put on the car.

Napier: I don’t think this is right, or at least think it’s too simple, if you’ll pardon the suggestion.

Yes, it was a simplification. It was in response to this in the OP: “Mom and I both do pretty strictly city driving, and not much of that - probably 2-3000 miles per year at most. I go on highways sometimes, but Mom never does. My Scion was designed for maximum interior room, rather than handling, but it does have power steering and some kind of special traction, as well as ABS. Mom’s car is just a little sub-compact.” The point was that all those things mentioned don’t really have any bearing on how Oy! should inflate his (her?) tires.

15

‘Her’, Gary T, not ‘his’.

Once again, thank you all. Obviously, underinflating (which is what I have until I can get some air :eek: ) is definitely not the way to go. Over-inflating slightly seems to have a little advantage to it, but not all that much.

16

Just out of curiosity, did you really need Obama to tell you to properly inflate your tires? You honestly never heard of the concept before?

17

I imagine my mom had, but I’m ashamed to admit that if I had heard of it, I had forgotten. :frowning:

18

>Just out of curiosity, did you really need Obama to tell you to properly inflate your tires? You honestly never heard of the concept before?

Well, pretty much everybody has heard of the concept of voting. But some elections are said to have been won by politicians motivating people to come out and do it.

And all of us here have heard of the concept of the WWW, too. But at least a few of us use it a bit more often because of the Board, eh?

19

The more apt analogy is that the OP hadn’t heard of voting but maybe her mother did. Voting is a new concept, introduced to a new generation of voters by Obama. Further discussion of the “concepts introduced by Obama” should be left to IMHO, I suppose.

20

Don’t get me wrong. I knew you were supposed to keep your tires properly inflated. I didn’t know it had a profound effect on gas mileage.