View Full Version : Tire Air Pressure

06-07-06, 12:05 AM
How much air pressure works best in stock 1993 Fleetwood tires 235-70-15 ? The door sticker says 30lbs. The tire store put 35 lbs air pressure, I let it out to 32 and am going to try that. I was running 32 in my last set of tires. I just want to know what pressure other are running in their tires. easymoney$$$$

06-07-06, 10:09 AM
35psi (or higher if the tires are rated for more psi) will give you the best fuel economy and load capacity and probably slightly more responsive handling, but at the expense of ride comfort.

30 psi is that Cadillac picked for their desired tradeoff between handling response, fuel economy, load capacity, and ride comfort.
I wouldn't run them any lower than that. Remember the Explorer tire thing? Ford had specified too low a pressure in order to try and make their big truck ride like a car and that was the result.

Because Cadillac specifies the same pressure front/rear, I wouldn't have any problem with increasing the pressure above 30psi right up to the max on the tire sidewall as long as you do it equally f/r.

If you are taking a long trip with the car loaded, I would definately run the tires as high as they are rated. The higher pressure reduces sidewall flexing as the tire rolls and this reduces heat buildup in the tire.

06-07-06, 11:54 AM
The door sticker tire pressure is not a "one pressure fits all" vallue. As I understnand it, that figure is based on the "recommended" presure for the factory stock size tires on stock size wheels (which you have mounted), when the vehicle was manufactured based on all around normal driving conditions. Tires even change over the years, so the recommended pressure for tires in say '95, may be a little different for tires manufactured in '06. I keep 32psi all around in my stock size tires. With radial tires, what you are striving for is even surface contact pressure across the tire tread. That will vary based on vehicle load, tire pressure, etc. The recommended pressure should give the best "overall" ability to maintain max. tread surface contact.

You may have seen illustrations of how a tire cups with underpressure and bows with overpressure. THe happy medium is what your want. As dificult as that is to evaluate, the manuf. provides us with a "recommended" pressure based on the tire size. Monitoring the tire tread for wear is about the best the average driver can do.

06-07-06, 02:17 PM
sculring is on the right track IMO.

All tires are different, and most likely you have different tires on your car now than what it came with from the factory. These may be a different brand by a diff tire maker, so the door sticker recommendation goes out the window pretty much....

Look on the actual tire you have and see what max PSI is molded into the tire. My 235/70-15s list as 44psi max load. I once heard a recommendation of 10-15% less than this max load is ideal. SO that is 38-40psi as a general rule of thumb.

As Blueyes said, different conditions and loads in the car will dictate what PSI you should run as well.

06-07-06, 03:21 PM
V4P is 30 lbs not towing, 35 towing. I run around 35 most of the time. I drop down to 32-33 if I feel the roads are harsh on me.

I will NOT go below 32. SCCA will not allow you to race below 32 due to the high risk of tire coming off the rim in emergency manuevers.

I race (cornering) with around 40-45, typically 4 psi delta front/rear.

07-01-06, 04:43 PM
thanks for all the advice. I will try to keep them between 32 and 35. The Vogue 235-70-15 tires have 35 psi on the sidewall.

07-01-06, 10:42 PM
The dealer where I got the new Coopers says run what's on the sidewall.
I did likewise on my Chrysler Cordoba, Gini's former Olds Delta 88, the old Dodge B250 Van, etc, etc..
That's what I keep on my wife's Dodge Stratus with Michelin Hydroedge tires. It hasn't bothered us nor passengers and that car has the optional hard suspension.
The ride of tires pumped to max to sidewall pressure may seem a bit stiff to some people, but it's a lot safer and easier on the tires and gas mileage.

I can't remember which outfit did it, but I believe it was some tire company ran a film that showed how badly a tire will TUCK UNDER in a turn when under-inflated. In an emergency situation, it's possible the tire might just come off the rim!

At a higher pressure the likelihood is almost nil that the tire will tuck under or come off the rim when pressed hard in a turn or slide. And ever since I saw that film I have kept 'em puffed up to what is shown on the sidewall.

07-02-06, 10:48 AM
The dealer where I got the new Coopers says run what's on the sidewall.

Probably doesn't apply to Cadillacs, but as I mentioned in my earleir post - be extremely careful doing this if the door sticker specifies a different pressure front and rear. Mostly the problem is with rear engined cars like old Beetles, Porsches and Corvairs where the different tire pressures are used to help tune the chassis balance and avoid terminal oversteer. 99% of cars should be fine running max pressure in the tire though.

07-03-06, 09:14 AM

I had my new 62 Corvair swap ends after I let Johnson Chevrolet, in Indianapolis, give it a lube job and I took a 45 degree corner too fast.

As I remember, there was a 10 or 12 pound differential - front to rear - in tire pressure and the kid had put something like 28 all around. So when I came off US52 on to 38th St at about 40, it did the damndest whoop-de-do you ever saw:eek:

Luckily there wasn't another car in sight:worship:

07-03-06, 11:32 AM
In my 91 Bonnie and 91 SDV, I would run around 38/34 or 40/36 most often. Being front wheelers are sooo nose heavy, having 6 psi delta front/rear is often fine. What I found in my 91 Bonnie is if I was only driving straight, GM's spec of 28 psi was close, sorta soft, but it would wear ok. But in any cornering it wasn't ok. Remember, front wheelers are light back ended. They are very nose heavy.

To PROPERLY determine your proper wheel pressure do the following proceedure:

1. Fill tires to GM spec as a baseline. COLD, meaning no driving, and tires not in the sun.

2. Get a piece of chalk (swip it from the kids, etc), on all 4 tires, mark 3 2-3 in wide chalk marks that go from around 1 inch up on the sidewall down over the shoulder of the tire onto the tread.

3. Drive your NORMAL drive. If you are going to do some racing or hard driving, do this separately. unless you drive like that all the time... Avoid puddles!

4. Come back and observer the chalk. Proper inflation should come to preciesly the end of the tread with NO loss of the chalk on the sidewall at all. If there is loss there, add 2 psi, rechalk and retest. Do this till all tires have proper wear and note that pressure once the tires cool. This is the PROPER air pressure. Note what temps you measure it at, and also note it cold. Write them down.

GM has to spec an air pressure that is good for economy, good for ride (which might comprimise economy!), good for tire wear, and be safe for most all driving conditions. GM seems to favor ride though.

As my V4P car specs 30 psi normal driving, 35 towing, that is one of the few that I have seen spec dual pressures. My 96 Suburban K1500 spec'd 35 psi. That is waaaaay to low. In 1998, same K1500 spec'd 45 psi. Same tires even. Same weight vehicle. GM learned from the Explorer fisako.

Now, my 1999 K2500 Sub is different, it specs 65 psi front, 80 psi rear (load range E tires), but has a non towing "soft ride" recommendation of 50 and 65 psi.

So in reality, tweak and adjust as driving conditions require. Recheck them often, watch the wear on the tread and note what it is doing and adjust as needed.

Note that tire pressure on the sidewall that says "MAX" people all see is for that pressure for that WEIGHT,, not max pressure of the tire. Some people think the tire will blow beyond that. No, it won't. Get up in the 50-60 range and you'll likely pop a belt though. Been there done that. RR tracks and high pressures didn't do me good. Think of the weight on the tire in a hard cornering manuever, it can increase a lot. So ensure the tire is up to the task!

07-03-06, 12:17 PM