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Let us start off with a fundamental truth; air in the tires, carries the car's weight. I recently bought new tires for my car. Typically, that results in a slightly improved ride quality (less "harsh" on sharp bumps). Got the tires installed in immediately noticed a harsher, worse ride. It was pretty bad and made the car not-fun to drive. My first reaction was that "these new tires SUCK!"...but they were the same brand and model as the previous, worn out tires that rode fine. I had previously been running my old tires at about 26-28 PSI, or so. I checked the tire pressure on these new tires and it was ~35 or so. Hmmm. I checked the spec on the door jamb and it called for 35 pounds. Really? 35?
I think 35 pounds for a C4, certainly a C4 equipped with a tire on the wider side, is inappropriate and here is why; Tire pressure needs to be high enough to support or carry the load. It has to be high enough that we don't excessively flex and build excessive heat in the carcass/side wall of the tire...but not so high that we're wearing the center of the tread or worse, stressing the tire's integrity or the rim. I believe that the correct pressure is a simple function of the tire size (width mostly) and weight; for a given weight, the larger the tire is, the less air pressure you need to carry the load.
My car has 265 fronts and 295 rear but originally has 275 all around ('92 base). Those are all very large tires (a lot of square inches of rubber on the road) for a 3300 lb car, yet the car calls for 35 pounds. That is too much. First, if you inflate to 35 two things happen with the tire itself;
1. You wear out the middle of the tire first
2. You don't have contact across the full width of the tire (you can observe this by wetting the tire w/a damp cloth, then driving it on a smooth surface -look at the tread mark left by the tire). You're running a 275 tire (or whatever) but only getting the contact patch of a 235 tire or so.
The second observation I've made is anecdotal; other cars. In comparison, my wife's V has 245 tires all around. It weighs 3800 lbs and also doesn't have a ~50/50 weight distribution like the 'Vette does; it's nose heavy with 54/46 so 3850*.54 = 2079 lbs on the front tires -compared to @1600 lbs on the front tires of the 'Vette. Know what the Caddy calls for tire pressure? 32 all around. Say what? Smaller tire, more weight but a lower pressure? Hmmm....
My Silverado. Weighs 5000 lbs. IDK the weight distribution but it's obviously front heavy. Weight on the 265 front tires is over 2500 lbs. Recommended tire pressure? 35 PSI. We've got a vehicle that places over 1000 lbs more weight on it's smaller front tires than my Corvette...yet calls for the same pressure.
My take away? The 35 PSI recommendation for the Corvette is way too high. You don't need that much pressure to carry that little weight, with that large a tire. I'm running 28 PSI in my new tires. Ride is great handling is great -the whole tire is touching the ground now and I'm not wearing out the middle of my tires.
Related story, my brother bought a new Pontiac from the dealer that rode horribly and sounded like the front end was going to rattle apart. The problem, the tires had been filled to 80 PSI for transportation and storage, but the dealer prep never let the extra pressure out before selling him the car.
80psi? I would have thought the tire would pop off the bead at 80psi.. I keep mine filled to 29-30psi filled with Nitrogen and keep a eye on it with my digital tpms readout.
Not 100% sure about this but I would think tires have changed a lot in 21+ years since the vette was manufactured so that could play a role in correct tire psi as well.
I keep mine between 30-35 cold for the last 15 years, Never seen any wear issues.
Really? I've never had a "wide tire'd car" NOT wear the center of the tire faster than the edges. I hadn't thought about pressures vs. weight and size until recently.
No, never seen any, and I've run from stock to modified-as-hell over the years, and ran all widths from 255 to 315. My wear has been pretty even, so when I see Abe's head on the penny I'm usually seeing the same amt between the center and the edge. I'm sure if you took a caliper and measured there would be some difference but overall I have never noticed one. I keep a pretty good eye on this as well.
Tire brands from Kumho, Dunlop, BFGs, and maybe another I can't remember. Kumho was the most sensitive to the pressure vs ride. The only ones I know I have never run on the corvettes are Michelins, Pirellis and Goodyears.
My Door Jam calls for 30psi front/rear for speeds below 150mph and, above 150mph 35psi for a 95! my ride feels pretty good, but I have been tempted to reduce the pressure by 2-3 lbs just to give it a try..
My Door Jam calls for 30psi front/rear for speeds below 150mph and, above 150mph 35psi for a 95! my ride feels pretty good, but I have been tempted to reduce the pressure by 2-3 lbs just to give it a try..
There we go. That's a number that makes more sense to me. Must be a later LT1? I see the staggered front/rear sizes...
My 35th Anni. has 275s around and I normally run 35psi around. At the strip I drop the rears to 30-32psi but find that 35psi gives best road handling in the twisties.
BTW, each set of GY eagles went over 50k miles with such tire pressure settings. And, no, the centers did not wear more than the sides.
Note that tires generally increase in their rated load capacity as you increase their inflation pressures, at least until your each the max rated load capacity. That's why space-saver spares use 50-60psi pressures. So yes, it makes sense that if you put wider tires on your car you should be running with a little bit less pressure than you would on narrower ones. Each brand of tire will be different in its ideal pressure on your car.
No tire manufacturer can tell you conclusively what pressure to run on your car. The pressure noted on their sidewalls is the max pressure they want you to use. It isn't a recommendation. The inflation pressures noted on the placards of your vehicle are focused more on liability avoidance than ideal ride or handling. Chevy cannot know what brand/model tire you have installed, so they can't suggest the best performing pressure for your car. And keep in mind there is likely to be a difference between the best grip, best handling balance, and best ride quality. There may also be a different pressure for best longitudinal acceleration (acceleration and braking). This is the kind of stuff that keeps race team crews employed!
Tom, my experience is that on our cars the rear ride quality is far more important to our perception of overall ride quality. With the 295s you installed, you probably do want to run at a lower pressure. Re the center wear, if you have access to a tire temp gauge you can test by driving in a straight line and check temps across the tread surface. If it's hotter in the middle, try dropping pressure until you get it close to even. This is another area where different brands models in the same size and on the same wheel will want different pressures.
The inflation pressures noted on the placards of your vehicle are focused more on liability avoidance than ideal ride or handling. Chevy cannot know what brand/model tire you have installed, so they can't suggest the best performing pressure for your car.
This makes the most sense of anything that I've read or heard. Thanks for posting that. I had thought that the data on the card was "gospel" -a result of...you know, "countless hours of engineering and testing" and it was the pressure that best met all GM's criteria (much like all the other compromises they make). I didn't realize, or maybe I had forgotten!) that it was more of a recommendation.
Originally Posted by MatthewMiller
And keep in mind there is likely to be a difference between the best grip, best handling balance, and best ride quality. There may also be a different pressure for best longitudinal acceleration (acceleration and braking).
I totally agree with this. Absolutely.
Originally Posted by MatthewMiller
Re the center wear, if you have access to a tire temp gauge you can test by driving in a straight line and check temps across the tread surface. If it's hotter in the middle, try dropping pressure until you get it close to even. This is another area where different brands models in the same size and on the same wheel will want different pressures.
This is a fantastic idea. I'll give that a shot one day and see what I find. Great idea....thanks!
This makes the most sense of anything that I've read or heard. Thanks for posting that. I had thought that the data on the card was "gospel" -a result of...you know, "countless hours of engineering and testing" and it was the pressure that best met all GM's criteria (much like all the other compromises they make).
Well, it is a result of that, but their goals are different than yours. They are mostly focused on load ratings and safety, not ride quality or lap times.
I should clarify one thing, though. IIRC (it's been decades since I looked at this stuff), load capacity is pretty standard for a given tire size regardless of brand or model. As long as two tires of the same size (not just width - see below) are in the same load rating category (e.g. A, B, C, etc), then they should have the same load capacity at the same inflation pressures. This is because the load capacity is based on air volume, and as you noted it's the air that carries the load. So the placard info does apply somewhat universally to tires of OE size and rating. That said, with your 295s you're outside their box and your tires will have higher capacity at the same inflation pressure than the OE-sized tires. Or conversely, you can run match the load capacity of the OE tire at a lower pressure for your wider tires.
BTW, in re-reading your first post, I noticed you mentioned your 265 truck tire being smaller but having a higher load capacity at similar pressures. Keep in mind that the load capacity is a function air volume, not just tire width. So I am sure that tire has much taller sidewalls than those of your C4 tires with resultant larger overall diameter, and it may also be a smaller diameter wheel. Both of those things increase the air volume in the tire. So width notwithstanding, your truck tires almost certainly have more air volume (probably almost twice as much). Therefore, they have a higher load capacity. Those truck tires also probably have a higher load rating class as well. So this is a bit of an apples:oranges situation.
This is a fantastic idea. I'll give that a shot one day and see what I find. Great idea....thanks!
BTW, even without a proper tire thermometer (that probes below the tread surface) you can probably still get useful data from cheap IR thermometer. If your 295s are on the OE wheels or on the same width as OE (was that 9"? 9.5"?), they will tend to bow outward at the center of the tread width a little more than stock at the same inflation pressure. So yes, all indications are that less pressure in your rears is a good idea.
On a related note, we sit nearly on top of our rear tires in our cars. The rear ride quality in general has a much, much bigger effect on our perceived overall ride quality than the fronts. So that's the end to focus on when adjusting things for ride quality.
I should clarify one thing, though. IIRC (it's been decades since I looked at this stuff), load capacity is pretty standard for a given tire size regardless of brand or model. As long as two tires of the same size (not just width - see below) are in the same load rating category (e.g. A, B, C, etc), then they should have the same load capacity at the same inflation pressures.
So check this out; I thought the SAME thing until I went to buy the tires for my car this most recent time (a few weeks ago). I called the place to make sure that they had the tires in stock and we chatted about the car. I explained that for sourcing the tire, use a '04 Z06 (where I got my wheels from)...but then when talking about the car, I mentioned that is was a '92. Well he informs me that the '92 needs a higher load capacity rated tire than the '04 Z06. Say what? They both weigh the same, have the same front/rear weight bias, very similar tire sizes and similar performance. I ended up having to take just the wheels down and have them mounted as they couldn't mount the car.
Originally Posted by MatthewMiller
BTW, in re-reading your first post, I noticed you mentioned your 265 truck tire being smaller but having a higher load capacity at similar pressures. Keep in mind that the load capacity is a function air volume, not just tire width. So I am sure that tire has much taller sidewalls than those of your C4 tires with resultant larger overall diameter, and it may also be a smaller diameter wheel. Both of those things increase the air volume in the tire. So width notwithstanding, your truck tires almost certainly have more air volume (probably almost twice as much). Therefore, they have a higher load capacity. Those truck tires also probably have a higher load rating class as well. So this is a bit of an apples:oranges situation.
I agree with all of this too. I through the truck in there but knew that it wasn't "proof" of anything, just an example to ponder. Not really relevant, though.
And keep in mind there is likely to be a difference between the best grip, best handling balance, and best ride quality. There may also be a different pressure for best longitudinal acceleration (acceleration and braking). This is the kind of stuff that keeps race team crews employed!
Lots of good information in this thread! I've experimented with different tire pressures on my '84, and find I get the best handling with 35 front, and 32 rear pressures. There's no reason front and rear need to be the same! They have different jobs to do, unless you never get off the freeway!
I have played around with wheels and tires quite a bit and there are a lot of things that influence what tire pressure to use. The big ones are the weight of the car and the size of the tire. Others are the wheel width vs the tire size, what tire performance you are looking for, tire sidewall construction, etc.
On most of my cars the tire pressures usually wind up at 30 to 32 psi for street use. I arrive at that by starting at a value (usually about 32 psi) and checking for contact patterns and after more time the wear patterns and make adjustments.
There are some extremes too. My S10 is very light in the back, so it wears the centers quickly unless the pressure is dropped. I went with bigger wheels and tires on it and it made it worse. I had to lower the pressure again. Another issue was when I went to 255/50/16 Michelin tires, they were so stiff that the truck rode bad and felt like it was skipping off of every bump. Lowering pressures did not help much. The tires were just too big and too stiff for that light weight.
Discount Tire Direct has given me problems about load rating and tire sizes several times. On my '95 they would not sell me a set of 275/40/17 tires, on my S10 they would not sell me 235/55/16 tires, on my '96 Caprice they would not sell me 235/70/15 tires, all because they said it was not the right size. In all cases I argued with them and they discovered they were wrong and these were the stock sizes. They also told me the load range was incorrect for the car and they needed the stock rating or greater to be safe even though I was buying a tire that was larger than what they were pushing. Their tire data is frequently wrong!
I was responsible for the engineering specifications and quality of the tires when I worked at an auto manufacturer, so I frequently don't agree with some of the things that are being said by the tires stores as fact. It is still hard to convince them.
Good luck with evaluating your tires and getting the pressures set.
For our Corvettes, I always understood the 35psi recommendation was only for high-speed and/or high-performance driving. But, doesn't higher pressure reduce the contact patch and therefore reduce performance? Doesn't make sense. At the same time it does make sense because higher pressure will reduce sidewall flex during performance maneuvers. Obviously I am confused.
Anyhow, my 1995 base new Continental XC DW tires (and alignment) a couple months ago. I had them up to 35 and viewing the side profile it looks like a tiny sliver of rubber is touching the pavement. So I go down a couple pounds 33psi all around, the ride is good and wear is even so far.
The first Corvette I bought was another 1995 and the seller had the tires up to 50 pounds in order to avoid flat spots. That car had the HD suspension and FX3 option and the high pressures made the test drive brutal, then I figured out what was going on and it was much better after letting out 15 pounds
I've never seen a driving situation where the maximum pressure (on the tire sidewall) was a good idea. Sure pump up the rears on a pickup if you are loading the bed, that makes sense. But for everyday driving, the tire manufacturer has no idea what model vehicle will receive their tires, what the weight distribution/tire load is, alignment settings, etc etc. (My Tacoma pickup takes the *exact* same size tires as my old 1992 Thunderbird.) If the tire manufacturer provides a recommended pressure based only on the type/size of the tire, they are making an educated guess. And if the maximum tire pressure is actually needed, it's possible that the vehicle is being used in a manner other than its intended purpose
11-16-2016, 12:18 PM
Ride quality and tire pressures....
There we go. That's a number that makes more sense to me. Must be a later LT1? I see the staggered front/rear sizes...
