Tires other than OE tires that came with car

I’ve always stuck by the recommended tire pressure (on the sticker from the car maker) on my cars over the years and stayed with the same size. My thinking has been that the same size tire in the same or very similar type and performance categories would use the same recommended pressure.

Today I heard that it is possible for new tires to require pressures higher than those on the sticker simply because they are not the same as the OE tires. Thinking about it, this seems to make sense because different tire brands and models may not have exactly the same characteristics even though they are the same size and type. So for the same weight load, maybe one tire is best at 33 psi while another is best at 38. Does this seem reasonable?

And if this is real, then anything but the original tires might be better at a pressure different from the sticker. And if true, I wouldn’t mind sorting this out for a car where the original tires are no longer available or a better tire has come along. And I’ve seen things like the chalk test to help test tire and car combinations.

But this makes me wonder how many aftermarket tires are in use with non-optimal pressures because the general rule is to go with the sticker, which may or may not have been optimal for the car in the first place!

A couple of items:

  1. The vehicle tire placard: Because tires are standardized, if you use the size written on the vehicle tire placard, then the pressure listed there is appropriate. It doesn’t matter who manufactured the tire. I go into more detail here:

Barry’s Tire Tech - Load Tables

  1. Tires do have different characteristics and you can vary those characteristics by varying the inflation pressure - BUT - remember a) the vehicle manufacturer set up the vehicle’s springs, shocks, and sway bar based on the spring rate of those OE tires and that spring rate is pretty much the same for a given inflation pressure (Put another way, inflation pressure is by far the largest contributor) and b) there are a whole lot of things that change when you change inflation pressure - some good, some bad.

Ergo, deviating from the vehicle tire placard must be done carefully and take into consideration ALL the parameters affected.

Always buy the same size tire and stay with the pressure listed on the vehicle placard. I add a couple of pounds to the recommended tire pressure but I never mess around with changing tire size.

Who told you that different pressures might be needed?

@Missileman and @CapriRacer are right.

Also, if you are in your 70s and remember the early tire pressures you should know that those like 24 psi were dangerously low but gave a soft pillow ride. Fords in particular in the past had tires that were really too small.

Most cars now have about 35 psi on the door post and you should use that for best results. Today’s tires are substantially larger than those in the past for a given with car, and 38 psi would give a had ride.

For instance, my 1988 Caprice and 1984 Impala rode on the same size tires 205/78/15 as my 2007 Corolla, which weighs nearly 1000 lbs less. Putting 38 psi in the Corolla’s tires gives a very hard ride, I use 34 psi as recommended.

As stated, the make of tire has nothing to do with the inflation; it is determined by the weight of the vehicle and the best overall compromise in ride and traction.

And if you owned an early Explorer… well,
{:slight_smile:

Seriously, unless you understand tire pressures and their effects, you’re best bet is to stick with the OEM tire size and use the pressures on the door jam sticker.

Most cars now have about 35 psi on the door post and you should use that for best results.

I’ve yet to own a vehicle in the past 30 years where the recommended psi is as high as 35. None higher then 32psi. Some as low as 28.

I do think going a little 2-3 psi over the recommended limit is good. But much over that and you’re asking for trouble.

But , since you can see the same tire…on different vehicles…having different service psi…then you should go by the placard for the vehicle the tires are on.
In the past when so many took the same size tires you could see the difference on an F150 vs a Ranger vs an Explorer.
different psi recommended on each.

Thanks to everyone for the comments. I know it isn’t easy to be clear sometimes, but I am trying.

I am not suggesting any changes in tire size. I am pointing out that tire technology does change and certain details are left for the general public (and me) to assume. So starting from the idea of the same size tire but different manufacturer or “tire model” I have the understanding that the ultimate goal is to have a tire that applies pressure evenly across the tread (from side to side) as the tire rolls forward. That’s why things like the chalk test can help, and why the test can confirm over inflation (so too much pressure on center of tread) and under inflation (more pressure on edges of tires) in a particular tire and car combination or tire and axle combination.

What I newly heard is that a different aftermarket tire (same size as OE) might not be flat at the pressure on the sticker/placard because the tires are different. This might be due to differences in the new tire (and new rubber composition, construction, whatever). And so going by the sticker recommended pressure is just an assumption. It might be the best assumption for most of the people most of the time. But it might still be an assumption.

In my case, it seems the pressure for some new tires needs to be a little higher than the sticker. I will be testing this out with chalk when I have the chance. This came from a reputable alignment and suspension shop that’s been around for over 30 years.

I’m sure they’re reputable, but the tire pressure impact of differences in radial tire construction for a given size would be pretty small, I’d think. Sure, might it vary a psi or two? Maybe, but that would be impossible to tell without a LOT of research. And adjustment in the pressure should only be UP, since load capacity will be compromised by any decrease.

And just because a shop does great alignments doesn’t mean they know about something as complex as the interaction between tire construction and optimal tire pressure.

The reality is that if you try to complicate the bejesus out of tire pressures, nobody is going to bother even trying. And I’m not convinced that doing other than sticking with OEM size tires and OEM recommended pressures will make any meaningful difference.

Yeah, I have my own preferred pressure settings. But the people who aren’t interested in this stuff should just go by the mfg’s recommendations. Suggesting they get complicated is folly.

@Mike My 1988 Caprice because of its skinny standard tires had 35 psi as the doorpost pressure.

Tire pressure is a direct relationship between the tire footprint and the weight of the vehicle. The more pressure you put in…the LESS of a footprint the tire will have. And visa-verse. In fact you can actually measure the vehicles weight by adding up the total footprint at all 4 wheels and the psi at all for wheels.

Mike in NH said: " … In fact you can actually measure the vehicles weight by adding up the total footprint at all 4 wheels and the psi at all for wheels. …"

Actually that is NOT a fact. It is a falsehood. I talk about that here:

http://www.barrystiretech.com/airortire.html

And to the OP, Waterbuff: At this point in the game, most tire manufacturers have developed enough expertise that their footprints are going to develop fairly even wear regardless. Any tire manufacturer that has problems in this area, is likely to be new to the business (as in a third world, third tier.)

The second part is that inflation pressure plays a minor role in evenness of wear. There are other things, such as alignment, that have a much greater effect. For example, the fronts of RWD vehicles tend to wear the shoulders and the rears tend to wear the centers.

Actually that is NOT a fact. It is a falsehood. I talk about that here:

Sorry…but Physics is physics. You CAN determine the weight of a vehicle by it’s footprint. The problem is determining it’s footprint. Tread pattern matters a lot. In fact several industries like Aviation and even the Navy uses that method.

We even did this in one of Physics classes almost 40 years ago. Tested it on 10 different cars…and the weight was pretty close.

http://illuminations.nctm.org/Lesson.aspx?id=3117

http://www.tandfonline.com/doi/abs/10.1076/vesd.39.2.135.14157?journalCode=nvsd20#preview

I’m with Mike on this one. The combined pressure with which the tires press on the macadam MUST equal the weight of the car. And the total weight MUST be distributed among the tires’ footprints. If you know the pressure in the tires, and you know the total area of the footprints, you can easily determine the car’s total weight.

As he said, however, tread patterns make accurate measurements extremely difficult if not impossible. The more aggressive the tread, the bigger the problem becomes. Slicks would allow an accurate footprint measurement… but be illegal on the road, of course.

I agree, except the sidewalls also support some of the weight. take a wheel and mounted tire with no air inside (ie, pressure equal to atmospheric) and with the valve removed place it on the ground. How hard do you have to push to get the tire to flatten so that the footprint equals that when on the car and under pressure? I never tried this but my guess is perhaps 50-100 pounds? Which would add 200-400 lb to the number.

So, if you let all the air out of your 4 run-flats, your car would weigh…nothing?

;-]

This discussion regarding tire pressure and footprint vs vehicle weight is interesting. And maybe it gets back to my original intent. Starting with the known that as pressure goes up, footprint decreases while low pressure means a bigger footprint, it still is based on the flexibility of the tire.

Now taking that to two tires, one OE and a different aftermarket, in the same size and to be used with the same vehicle weight, it seems possible for different rubber and composition between the two tires to mean that the same footprint might require different pressures. This doesn’t mean drastically different pressures, but within 5 psi or so doesn’t seem outrageous.

Of course of the tires are identical or nearly so in materials, then there might be no difference in psi for the same footprint. On the other hand, same size tires with maximum pressures that differ by 10 psi or so, or have very different maximum loads, would seem to suggest that a single recommended pressure might not be right for all tires.

But how would you determine the ‘right’ pressure?