Another tire pressure question

I know To use the manufactures recommended tire pressure on the door. My 75 Vette has a recommendation of 20 psi front and 20 psi rear. It is what I have always used but doesn’t that seem mighty low? I checked with a Corvette specialist shop and they said use 30 psi and 30 psi but no explaination of why. Any thoughts on this?

That does seem awfully low. What size are the tires, and are they radial or bias ply?

The rag tires (2-ply polyester) that came on 1975 model cars wore out in 8-10K miles no matter what, so why not enjoy a nice smooth ride? Nobody paid any attention to “manufactures recommendations” back then. Very few “Smog 'Vetts” were sold '74-'81. Doctors and Lawyers bought them because they looked cool…

I’m also wondering of the 'Vette came with bias ply tires. Times have changed.

The manufacturer probably set the pressure so low to please the legal department by reducing the cornering grip in an attempt to make spin outs and rollovers less likely. Reminds me of the Ford Explorers with the Firestone tires set to marginally low pressures.

Radials had taken over passenger cars by the early '70s.

“Radials had taken over passenger cars by the early '70s.” NOT!!

You were lucky if you could get GoodYear Polyglas bias-belted tires in 1975. The “Wide-Ovals” that came on Vetts were still 2-ply polyester…F70X14 something like that…

Normally I say stick with the manufacturer’s recommendation, but as noted, that recommendation was for different tyres. I would suggest that the 30 psi sounds about right.

I now have Goodyear Eagle GT II P255/60R15 on it.

They are COOL!!!

28-30 for “normal” driving, 36 for “sport” driving…

I am a woman… but I could understand if someone gave me anything other than, “Oh put 30x30psi.” Why that number? When I first ask I was told that 75 Vettes did not have 20x20psi on the door. Why do you suppose I would make that up? Of course that is what is on the door. I neglected to say the tires are radial but I assume all Goodyear Eagle GT II are radial.

As long as you don’t try to drag race someone in a modern family sedan. :slight_smile: Those Vettes weren’t exactly quick. A former boss of mine has a 1974 Vette with the 454 big block and automatic. It’s not nearly as quick as you would expect it to be thanks to the smog gear and low compression. My Mustang smokes it easily.

According to Tire Guides, a 1975 Chevy Corvette came with GR70-15’s on 8" rims inflated to 20 psi front and rear.

That means the original tires were indeed radials, but you have installed a different tire size - and that means the pressure listed on the placard is no longer appropriate, so it needs to change as well.

At 20 psi, a GR70-15 has a load carrying capacity of 1180#.

To get the same load carrying capacity, a P255/60R15 needs … ah … well … the chart doesn’t go that low. The lowest value is 26 psi, which is also, the minimum pressure I recommend anyone use for street uage. This is because it is possible to generate enough sideforce to unseat a bead with the tire inflated below 26 psi. (It’s possible to generate enough sideforce above 26 psi, but the risk is much lower!)

CapriRacer, I am impressed and informed!!! Thank you!!! I will go with the majority at 30 x 30 but I just wanted to know why and that certainly makes sense. Now… I called the local Mercedes dealer to ask about the strange 29 psi front and 35 psi rear on my 560 SL. I have been using these numbers for years but after checking air in all my car tires I neesed to know why all the differences. The MB service department man told me that these figures are correct but to use 30 not 29 front and 34 not 35 rear. WHAT??? I acually said ,“Thank you very much, Good Bye.” I don’t know if I am that accurate - a pound? The tires are all the same. Again…Why all the discrepancy and general guessing about these numbers? CapriRacer you’re my HERO today!!!

Well, FoDaddy… you found me out! I am a retired school marm. Speed is not what I was looking for. You are exactly right it is not quick but… as you zip by I’m still at the light gettin’ the looks. ;0)

The factory recommended pressures are an average established at the time for the tires available then. You would not want the OEM tires on this car today, they would greatly disappoint you compared to todays tires.

With change in tire technology, there is bound to be changes in the best tire pressure for the vehicle. But the ideal tire pressure is really an individual thing based on your driving style and the tires you currently use.

The best guide to the best tire pressure would be the wear patterns on the previous set of tires and then adjusted to the wear patterns developing on your new tires. Few people go through the trouble of checking this, but the rewards are impressive. I’ve gone over 100k on the last two sets of tires I’ve bought and I am a rather aggressive driver, especially in corners.

You need a tire tread depth gauge to check this. They only cost a couple of bucks, I think mine was $2, but that was many years ago. I also have a good battery powered tire pump.

Once every couple of months, I go out early in the morning and check all my tires. I check for any abnormal wear patterns and for even tread depth on all the tires. Then I check the air pressure and refill any that need it to the air pressure that works for me.

I also do not rotate my tires on a RWD vehicle and I do one rotation, front to rear only, on my FWD vehicles. When I had a 4wd, I did rotate, front to back whenever the difference between the fronts and rears were 2/32" or more. That 4wd was a Tercel 4wd wagon so the 2wd mode was FWD, not RWD as most 4wd vehicles are.

The reason for the limited rotation is so that I can spot any alignment issues. Frequent rotation can mask these issues so they never get corrected. With FWD, the front tires wear a lot faster than the rears so I do the one rotation when the fronts are down to about 5/32". That way they all wear out about the same time and I replace all four then.