Tire pressure

Last Friday I put a set of Goodyear Comfort Treads on my 98 OBLW. The tire dealer filled them to 29 psi as spec'd on the door sticker, but the tires looked really flat at that pressure. The maximum recommended pressure on the sidewall is 44 psi but the dealer said that would completely wrong for the car. That got me to thinking.

I reckon that the tire is designed to hold a certain shape under load, and depending on the weight of the car that would involve different pressures for different cars, and the maximum on the sidewall would refer to the pressure at the highest rated load. Different tires also have different sidewall stiffnesses, so it doesn't make sense to go by the OEM door sticker when changing to a different brand/model tire. If I put too much pressure in the tire it will have the wrong shape and not handle correctly. But I like to have the highest reasonable pressure to decrease rolling resistance, since most of my miles are highway.

Is there a way to calculate the correct pressure for a specific tire and car weight (assuming typical passenger/cargo load and driving conditions)?

Reply to
BobN
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I've got the Goodyear Triple Treads on my 04 Forester and I run them at ~

34-36psi. This is a good compromise between fuel economy and comfort. The lower oem suggested pressure would give you greater comfort (ie: not as jittery) over bumps etc, but sacrifices handling and fuel economy. Greater than 40psi would probably be too coarse of a ride for most drivers. I assume that quiet and comfort are important to you based on your tire choice. Try 34psi and adjust from there. You'll know when it's too much when the wheels feel like solid rubber over railroad tracks etc.
Reply to
Grolsch

I wonder if Goodyear has recommendations other than the max pressure?

Reply to
John Rethorst

Generally the manufactures recommended tire pressure is the all around best for safety, comfort and life. I do run my Forester at ~ 2 psig over the door. One reason manufactures pick the pressure is to provide the driver with some advanced warning when the tire is starting to lose traction. To date Ford is the only big screw-up by recommending a pressure lower than their own and Firestones engineers recommendation on the Explorer and we know what happened to the rolly polly Ford when a blowout occurred.

Reply to
Edward Hayes

Reply to
Tcassette

Currently I'm running the tires at 36 psi, and they feel fine to me. A firmer feel with easy rolling is more to my liking, if I wanted a cushy ride I'd have kept my Cadillac. I'm going to try emailing Goodyear and will report back if I get a useful reply.

Reply to
BobN

I suspect the dealer fill the tires during the hot part of the day. I would refill the tires after sitting overnight using a good gage like a digital.

Reply to
Edward Hayes

About a year or two back I ran my tire pressure right at the 44psi upper limit, because I had found some of my tires had lost their shape after I went out of the country for a month. I was trying to use the higher pressure to reshape them back to shape. I didn't find that the tires were any less grippy at maximum pressure either on dry or wet roads. Mind you, I hardly take them to their limits most of the time, but neither do most people.

Yousuf Khan

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Reply to
Yousuf Khan

I don't know of any method for calculating tyre pressure ratios. However, I experienced severe wear in the Yokohamas originally supplied with my 05 OBW. I too was using the recommended tyre pressure until someone in this group suggested increasing. I did this up to almost maximum, less about 5 psi and the wear in the tyres has decreased markedly. I don't explain 'em, I just does it!!! Good luck.

Reply to
Phil

I thought it was common knowledge that as long as the tires you are replacing are the SAME SIZE as the new ones, the automobile manufacturer's inflation recommendations are the ones to use. The car maker's engineer's have determined based on the best combination of handling, fuel economy, ride, tire wear, etc.. what those inflation pressures should be. Sure, one can inflate lower or higher to suit one's taste but remember this: someone who's sole job was to make that determination probably went to a lot of trouble to come up with the right number.

Reply to
Googler

Famous last words - 2nd only to "Hey watch this..." :-) :-)

The maximum mainly referes to bead tension - i.e. the maximum pressure at which the tire will reliably stay on the rim.

You are forgetting that 99.99...% of the weight of the car is supported by the air in the tire. The side walls do little beside keeping the air in. Hence tire air pressure is 99.99...% a fuction of vehicle weight, not tire construction. The only effect that tire construction will have is in the volume of air containted, which should be constant for all tires of a given size.

You are assuming that you know what shape the tire designer intended the tire to be in.

cheers!

Reply to
Dominic Richens

I just put Triple Treads on my '03 WRX Wagon. Recently checked my tire pressure. Finding them around 35 psi, which would have been done at install, I filled them to 40 psi (they max at 44 psi) and I'm much happier with the ride and the handling.

YYMV

MB

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Reply to
M. Brumbaugh

You won't be much happier with the tire wear at 40 PSI

Reply to
JD

The OEM tire pressures are based on the OEM tire ratiing (ie. S, H, etc) and the weight and balance of the car and suspension. If you are going to a higher speed rating, the sidewall will be stiffer and you can get away with lower or slightly greater pressure.

Just remember that too low pressure is a problem because of heat build-up. Higher pressure deforms the tire laterally putting more pressure on the road in the centre of the tire. That will cause the tire to fail quicker, because you may have plenty of tread on the edges, and be to the belts in the middle. The tire should look slightly flat and the contact patch should be as flat as possible longitudinally as well as laterally. If you over inflate them, you actually reduce the size of the contact patch and your traction as well. If you want to inflate them to high pressure (for handling), you are better off getting a bigger wheel with a lower profile that increasing the tire pressure.

Reply to
JD

Hi,

As a normal practice with things automotive, I've learned the engineers generally know what they're talking about, and most attempts to "improve" upon something may actually improve one aspect of performance, while compromising another. Unfortunately, the engineers are tasked with designing a product that makes the most people happiest the most time. Which means there's always gonna be something that each of us as individuals will like or dislike more than another might.

Which leads us to tires...

For my own uses, I've always found it's beneficial to use the door sticker as the "minimum" and some door stickers allow more leeway than others. For example, my older Subie says 28/28 psi frt/rr w/ light loads, 28/32 psi frt/rr w/ full loads, yet the gross axle weights are calculated at 36/36 psi frt/rr. So there's some room for experimentation in there, and I've found 32/32 to 34/32 frt/rr gives me the best combo of feel, fuel economy and tire mileage under MY driving conditions.

Even discounting our individual driving differences, there's temperature. Some areas experience rather constant temps for reasonable time periods. But here in the part of SoCal where I am, it can easily be

35 deg F one morning, 55 or 60 the next. So if one inflates the tires to the door figure on a warmer morning, the next morning they could be "low." To help compensate, I've been told to "always" go at least 2 psi over the door sticker as a "starting point" by numerous tire dealers over the years, and, like Ed H, find that good advice. Then one can fine tune as desired.

Regardless, I'm in agreement with other posters a GOOD gauge is mandatory! I've used digitals and have an Accutire, which is a nice one, but prefer the Accu-Gage analog cuz it's easier to bleed off excess pressure and match the tires. Remember, too, even the best guages have a little leeway (+/- 1 or 2% and up depending on pressure) in their accuracy--another reason to go a bit over sticker pressure.

Rick

Reply to
Rick Courtright

if your using the tires the engineers designed too, and using the rims the engineers designed too, and suspension, etc., then yes. If you start changing things then no.

Reply to
CL (dnoyeB) Gilbert

Still - it's usually a one size fits all proposition. I filled my 2004 WRX tires to 4-5 PSI over the door jamb numbers with Pirelli P Zero Nero M+S tires. I'm finding that there seems to be a bit more wear down the center of the tires. I'm trying a little bit less - maybe 2 PSI over the factory recs.

The problem is that the number on the door jamb is a compromise. It won't always account for different tires of the same size, load, etc. Too little pressure and the tires run hot. Too much pressure actually makes handling "jittery". It's really a starting point more than anything else.

Also - there seems to be an assumption that the number was precisely determined. I find that sometimes engineers come up with an acceptable range, but then marketing takes over to try to make the product more palatable.

Reply to
y_p_w

I have three tire gauges, one digital and 2 analog. I check the tires in the garage, cold, about every two weeks. The first tire, I use all three gauges to make sure that they all agree. The rest of the tires on both vehicles I just use the digital gauge because it is easier. For the last several years, all three gauges have agreed, so I have no problem. I still haven't decided what I will do when they disagree. It depends on whether two agree and one disagrees, or if all three have different pressures. I'll decide when that day comes.

Don

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Reply to
Pinehollow

What this guy said.

The sticker is for the tires the car came with. How can they apply that psi to a tire which came out later? What form of time travel do the engineers have, that they can fortell what tires are like years later?

Compare Max psi ratings on the old and new tires. Are they the same upper limit? No?

Hmmmm....

TBerk

Reply to
TBerk

ROFL :)

Reply to
CL (dnoyeB) Gilbert

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