Yellow6t7

Can someone help? When using radial replacement tires in lieu of original equipment tires on my 1967 Corvette what air pressure is recommended.

Vetterodder

I'd start with 32 psi all around but the "right" pressures may be very different from that. Generally, a larger (volume wise) tire needs less pressure than a smaller tire to carry the same load or to give the same contact patch. There are advantages to both higher or lower pressures depending on the individual tires, the application, and their usage.

Yellow6t7

Thanks, I did set the tire pressure at 32psi. I was thinking of trying 30 psi to soften the ride. The original tire would be set at 24 psi. Any thoughts?

SWCDuke

The OEM 6.70-15s and 7.75-15s were speced at 24 psi for a soft ride. Assuming you run 205/65R-15s or 215/70s you could run as low as 24 since these tires have more load capacity than the 6.70s or 7.75s, but at that low a pressure the car won't have as good steering response or handling as 32 psi.

In any event, if you take a long highway trip, especially in warm to hot weather, I would run 32 psi. I also recommend the highest speed rating you can find, and there is one 215/70 that is "W" rated - 168 MPH.

Higher speed rated tires have more heat resistance. Lower pressures allow more flex, which builds heat in the tire at the expense of fuel economy. The reason SUV tires disintigrate is that many are not speed rated and the manufacturers specify low pressures to get a decent ride with their crude truck suspensions. Non-speed rated tires only have to be tested in the laboratory at design load and pressure to 85 MPH.

Duke




[Modified by SWCDuke, 10:10 PM 11/15/2001]

Tom/99

I am running 35 psi. in the 195 X 15 GoodYear radials, on my 65 Coupe. They are looking good with about 8,000 miles on them. :chevy

stngry63

I'm currently running 36psi front/34psi rear. The handling is quite a bit better, but the ride is harsher. I've run 46psi/44psi at an autocross. Poor traction; great handling; probably dangerous on the street; jars your fillings :crazy: For long highway trips, 32psi gives a decent ride. 32 is what the installer put in. The car has 215/70-15 Dunlop tires. I would highly recommend new gas shocks. They noticeably improve the ride and handling. I have some off-brand gas shocks that work great.

396 RAT

It depends on the tire!
What does it say in the fine print on the tire?
My Mich MXV4's require 42-44 lbs.
And they look the same when they are low,
drive about the same in a straight line or cruising.
But will not act right in certain situations.
(Like a 4 wheel drift with low pr. will make your car handle in a way
that will surprise the heck out of you! :eek: )
Trust me on that one............. :rolleyes:

JohnZ

The "fine print" on the tire indicates the maximum pressure the tire can tolerate without damaging the tire; normal operating pressure is considerably lower, and is specified by the vehicle manufacturer to satisfy their ride and handling objectives with their vehicle weight and suspension design. 32 psi is generally a good starting point for normal driving with radials, with front/rear differences a matter of tuning for improved limit handling.

SWCDuke

The pressure placarded on the sidewal is the MAXIMUM cold pressure along with the maximum load the tire is capable of carrying at this pressure. THIS IS RARELY the pressure you should run on the car.

As pressure decreases, the allowable load decreases. The OEMs have that data, but it's not usually available to us. They use this data to come up with pressure recommendations for the vehicle. For example, the C5 run flats are recommended at 30 psi cold versus a maximum placarded pressure of 44 psi, and it turns out 30-32 psi (cold) is optimum for handling.

Notice that if you add up the maximum load capacity of all four tires, you come up with about 7000 pounds - almost double the gross weight of a Corvette . That's why you usually run lower than the maximum placarded pressure. It won't harm to run it this high. You'll get better fuel economy, but the ride will be harsh, and the tire will likely not have optimum traction and cornering adhesion with the relatively low tire load (vehicle weight).

All tires are placarded with a maximum load and cold pressure, but this maximum cold pressure is rarely the "correct" pressure to use for everything from cruising to hot lapping race tracks.

Duke

StrayDog

28 all around ...I have radials 205 X 6 5 R 15"............for 5 years, this has worked out as best for ride & handling ..at least in my situation......

396 RAT

The jump from 30-32 to 42-44 occured a few yrs ago with newer tire designs
on some tires. (I thought)
It was my understanding these 2 types of tires are not the same.
32 lbs in these tires makes the car corner like jello. And is dangerous IMHO.
This is just my personal observation.

SWCDuke

It depends on the tire as there are several different systems. "P-metric" tires, those whose size designation begins with a "P" are typically placarded at 35 psi, but some with high speed ratings may be more.

Non-P-metric tires are typically placarded at 36 or 44 psi max cold pressure.

Back in 1964 I replaced the OEM General Jet Aires on my SWC that I chucked while hot lapping at Kent with a set of 6.70-15 Michelin-X radials. This was long before there were maximum pressures placarded on sidewalls. Because those tires were very high profile the sidewalls would roll quite a bit in hard cornering. I experimented around with different pressures and finally went with 45/50 front/rear. The limit handling was better, and those suckers rolled for 47K miles including several more track sessions.

In '68 when I mounted fabric belted 205HR-15 fabric belted CN-72 Pirellis on seven inch wheels (later replaced by 225/70VR-15 CN 73s) I found that 32 psi cold was about optimum for handling on both those combinations.

High pressures will usually improve handling at the expense of ride harshness. Everyone has to figure out what is best for their needs, but you should not go lower than the OEM recommended 24 psi, nor higher than the maximum placarded on the sidewall.

For modern radial tires on a Corvette 32 psi is probably a good place to start. From there you can experiment to find what best suits your driving style and priorities.

Duke


[Modified by SWCDuke, 7:00 PM 11/17/2001]

magicv8

My personal rule of thumb for optimizing mileage, high speed manuverability, and wear of the edges of the tread is to inflate to at least within 2 psi of the maximum stated on the sidewall of the tire. The high pressure also minimizes heat due to flexing that will cause tread separation and other catastrophic failures.

That having been said, I will state my favorite remark of the last 40 years about how hard I like my cars to ride: "Put a dime on the road, and when I run over it - I'll tell you whether it's heads or tails."

Vetterodder

As Duke has said, the max pressure listed on the sidewall is not a recommendation, it is a warning not to exceed that presure. As already posted, the air pressure used affects several areas of a tires performance. The most basic consideration is load capacity and a tire's load capacity is a function of the amount of air it contains. At a given volume, i.e., for a certain tire size, higher air pressure results in more air inside the tire and therefore the ability to carry more weight. Many truck tires are rated for over 50 psi. Also, as already posted, more pressure stiffens the tire (the Viagra effect? :)) and that, due to less sidewall movement, usually results in quicker response to steering input and also less rolling resistance. Lower pressures soften the sidewall and usually give a softer ride but slower steering responses and create more rolling resistance. Changes in pressure also affect the contact patch of the tread. More pressure forces the center of the tread against the road, less pressure forces the tire to ride more on the outer part of the tread. Either scenario, too little or too much pressure, will result in less than optimum contact and premature wear. Another issue is that different brands, and often different tires of the same brand, even though the same exact size and rating, may respond differently to the same air pressure. In any case, there is no one pressure that's right for everyone and no great harm will come from using pressures that fall within the min/max recommendations.

SWCDuke

A couple of other notes. Modern low profile tires generally work best with relatively low pressures, and old high profile designs seemed to like rather high pressures for best handling.

Current Indy car and F1 tires are usually filled to only about 15 to 20 psi ,cold.

You should always check and adjust your tires when they are cold. Sometimes I overfill them when they are hot at a gas station on my way home and then bleed them down the next morning to my desired settings.

Pressure increases with temperature, so after a hour of driving they are usually up several psi, and they should never be bleed down. Hot lapping a racetrack on a warm to hot day typically increases the pressure by about 8 to 12 psi over a cold pressure setting in the mid thirties.

Duke