Tire Pressure Recommendation

(Mark Gordon) #1

I just put on a new set of tires, Universal Sport Classics 185 HR 15, replacing the same tire that I’ve been running for the past several years. The maximum pressure rating on the side wall is 35 psi. I was running 28 psi front, 32 rear and the tires all wore out in the center of the tread, which to me would indicate over inflation. Longstone in England has a suggested inflation chart and various brands of that size recommend 32 psi for some makes and up to 40 psi for others. From that, I am inferring that the recommended pressure would depend on which brand and how the tire is used (high speed, aggressive driving, etc.). My driving habits are generally within 5 mph or so of the legal limit with little aggressive driving. I’m looking for optimum tread wear while maintaining the handling that the E Type is designed for. What is the opinion of the forum on where I should start? Maybe 28 front, 24 rear? I know that under inflation can lead to side wall fatigue and potentially failure, so I certainly don’t want to risk that.

(David Divins 1970 S2 2+2) #2

I run 32 all around on my Vreds.

For gentle street use, I think you’re overthinking it :slight_smile:


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(Harvey Ferris) #3

32 psi is kind of my “set it and forget it” tire pressure for all my cars, including the Etype. If you are wearing out the centers of the tires at the pressures you mention, I would question the calibration on your tire gauge.

Also, since the Etype has more weight on the front axle than the rear, I probably wouldn’t have run the front lower than the rear. You are right, over inflation might be indicative of wear in the center of the tread but I would consider over inflation to be numbers north of 40 psi. You’ve got something else going on here.

A very crude check is to use white shoe polish and make some stripes across the width of the tread and maybe an inch or so up the sidewall. This will give you a wear pattern. For track use, we do this to see how much the tire is rolling over on the sidewall but it may give you some indication of issues with the way your tires are loading for normal street use.

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(L.Lynn '68 OTS, '73 2+2) #4

Good Morning Mark,
I’ll add my name to the list of folks who run their tires at 32 psi front and rear. I think that this would be an excellent starting point for you and then you could adjust them up and down on the front to suit your driving style. That is assuming that your not planning on extended high speed driving then I’d defer to others.

(Mark Gordon) #5

Thanks for the replies. I had talked to the tire supplier’s customer service rep. and asked him this question. He looked up the answer and told me 35 psi. I said that that is the maximum pressure for the maximum load. He then said that he would recommend 32 psi. I think that he was just flying by the seat of his pants with his answer. I would agree with the general consensus that 32 psi is about right, but it still doesn’t explain my middle of the tread wear pattern. Other than the wear pattern, I have no complaints about ride or handling. I guess that I’ll just go with 32 psi and live with the wear.

(Les Halls 1968 S1.5 2+2 Atlanta) #6

“…it still doesn’t explain my middle of the tread wear pattern…”

Mark. are you 1) in a location where the average temps are hot…and 2) are you setting the pressures with the tires cold?

If your location is very hot, and you set the pressures when the tires are cold (correct way) then it might well be that the tires get warm enough that the pressures go up high enough to cause the center tread wear.

I have a recent example of this…my wife was on a long trip and called me to say the TPI on the car had come on. She took pics of the four corners and msg them to me, they looked ok and the car, to her, seemed to drive ok. When she got home I immediately checked the pressures and recalled the the dealer had set the pressures to 38, book called for 36, but when hot and when I checked them the tires had reached 41.5psi…hence the TPI coming on. Needless to say I waited for the tires to get cold, the next morning, checked them, they were 38 cold, and set them back to 36. Surprising what a couple of PSI can do when road temps get very hot.

(69 FHC ) #7

I inflated the tires on the car to 34 psi for an 800 mile road trip last year. I liked the feel and steering effort so that’s where I keep mine.

(Mark Gordon) #8

Good thought, Les, but no, I always check and adjust tire pressures while cold. I live in suburban DC where summer temperatures rarely exceed 100 F. (normal July-August highs are around 90 or so) and rarely go below 20 F. in the winter. And I don’t have any unusual wear patterns on our two daily drivers with which I use the same method of cold checking.

(Harvey Ferris) #9

You said “all” the tires wore in the middle. To me that would rule out alignment issues. Maybe you fell into a bad batch of tires. Stranger things have happened.

(Larry velk) #10

Not getting into the tin foil hat conspiracy angle, but what is the objective of tire and auto manufacturer advisories? I maintain they figure what is best for safety and performance and then bias the advice to the “safe” side of any spec. Generally (in other words, not always) tires run cooler and have more weight capacity at higher (not highest) pressures. My take on this is that I run lower pressures and our cars run smoother and the tire wear is perfect - I do my own alignments. Not a lot lower, but I do run 32 on the G37sx, as the warning comes on at 26. Nissan apparently thinks 26 is too low - for real. Have the huge 205/70r15’s at 29 psi as an “E” is, what, 2800 lbs? Infinity is 3600+, granted, different size tire, but different size car with tires designed for a Crown Vic or …(Like Longstone says… not that that would be gospel from the mount…).
My Dunlops were recalled because they came apart at low pressures, high load and high heat. So low pressures not the safe side by experience. Therefore my thinking that the recommendations are for higher than absolutely ideal. Err on the safe side.

(Harvey Ferris) #11

Here’s another way to look at it:
If the car weighs 3200 pounds, then there is 800 pounds on each tire. If the tire is inflated to 32 psi, then the contact patch will be 25 in2. If the tire is 5" wide, then the tire has to “squish” to a flat spot that is 5" wide. So you’ve taken a perfectly round tire and put a 5" wide flat spot on it. As you roll down the road, you don’t feel a thing because the side wall of the tire is constantly flexing to compensate. But that sidewall flexing builds up heat. As you lower the pressure, the flat spot only gets bigger and the tire is working even harder to adjust for the flat spot. I’m not a tire engineer, just a nuclear engineer, so don’t take this as gospel but I suspect as you run with lower and lower pressures, the tire will either overheat and fail or it will pop off the rim. And this is in abscense of shock loads like when you hit a big ole pothole. So yes, the manufacturers have to strive for a compromise of best safety and performance. No conspiracy, it’s just what we engineers do :slight_smile:

(Bob Curran) #12

I am firmly in the 32 lbs. front and rear camp. As a general rule, tire pressures go down as the size of the tire goes up. Bicycle tires are commonly inflated to 120 lbs. A large tractor tire might be at 6 lbs. There is a school of thought that higher is always better, but it will alter the way the tire rolls, corners and wears. If all four tires on a sports car wore out prematurely in the center, I would say the pressure was significantly higher than appropriate for that tire.

(Larry velk) #13

I was joking about a conspiracy, just saying they do as you say, compromise, then add a seat of the pants safety margin. Taking into account the end user and the real risks. Chevy specified very low front pressures on the first Corvairs, forgetting (or overestimating) the attention of owners. Handling suffered. Also, as far as low pressure causing heat build up - not at 29 psi and a 2800 lb. car. Twenty nine isn’t “low” in the sense that it would cause an issue in street use with a 205/70r15 donut.
There was a physics lab experiment which had students do a print of a tire, allow for the air pressure, calculate the area based on the print, and calculate the weight of the car. Never had my students do it, but it was in the PSSC lab manual (produced due to 'Sputnik" by the US to improve education).

(Mark Gordon) #14

That’s a possibility that I hadn’t considered. I will check my gauge against another one. I think that I may have found the answer, though. I took a careful look at the new tires and noticed that the tread depth at the center of the tire is shallower than the tread towards the sides. I measured the center depth at 0.255" while the rest of the depths come in at around at 0.325". What appeared to be excessive wear at the center was really just the tread depth being 0.070" less than the rest of the grooves. I checked my spare which is a Vredestein 185x15, never been used, and the tread depth is uniform all the way across. I wish that I had access to those old tires, but they’re gone by now. I wonder if this design is unique to the Universal Sport Classic or if any other brands have that shallow center tread depth. At any rate, 32 psi it is and I’ll just stop worrying.

(Robin O'Connor) #15

Interesting, marketing ploy??
‘Well sir you must have a problem with your cars alignment/pressure if the tyres are wearing out on the crown’ states the tyre rep.

A couple of instances come to mind.
1, smart ass states that he can increase the sales of tooth paste by ‘x’ amount at almost zero cost and no additional advertising, open up the throat of the toothpaste tube.

2, partial to Toblerone chocolate (the triangle) I used to place a normal dinner knife handle between the peaks and separate them, suddenly I had to use a carving knife handle. The B@£(*&()s had remade all the moulds with larger segments, same weight but probably lost 2 or 3 segments, ergo more sales.

Conspiracy anybody :slight_smile:

(Harvey Ferris) #16

Understand, joking about the conspiracy!

They are doing some innovative things with tires these days. I understand they are able to lay in different material recipes across the width of the tire, so that the middle is tough for mileage and the edges are sticky for cornering. I wouldn’t be surprised if the tread depth isn’t part of their science.
I met a guy at a track event that worked for Michelin in South Carolina. I asked about a factory tour. He said no way, they are extremely protective of their trade secrets.

(Geo Hahn 1969 Series 2 OTS) #17

You mean you would prefer a set of Vreds 185HR15?


JEGS will match whatever the lowest price is that you can find.

(Mark Gordon) #18

I probably would have bough Vreds several years ago if they made a narrow white stripe which I think looks good on a cream OTS. Vreds are considerably cheaper than Universal Sport classics and from what I’ve read on the forum, guys that have them really like the handling. I’ve seen cream OTS’s with black walls and, to my eye, they just don’t look as nice. I’m sure that some will disagree with me and that’s fine; that’s why since Henry Ford got started, manufacturers offer their cars in more than one color.