This is the closest to not being a WAG in the whole thread. IF you know the corner weights or anything close a tire co will shoot a 'starting point' for a performance level. They do answer the phone just make it quick and simple.

 

Within the same size, different tire models still have various load ranges, and those are standardized classes of load capability. You'll find that they generally correspond to higher max tire pressure ratings. This has to do with carcass and bead construction. Here is a chart and page that gives some details: http://www.tirerack.com/tires/tirete....jsp?techid=55. So it is possible that some tires of the correct size won't have a high enough load rating to qualify as proper fitment for a vehicle. The required load rating isn't about the empty weight either, but the gross vehicle weight rating given by the manufacturer. So maybe your C4 has a higher GVWR than your Z06?

For a given tire on a certain size rim and on a certain car, there is exactly one pressure that will provide the best grip. Going either above or below that will reduce grip. Often, but not always, that ideal pressure is higher than most of us run on the street. It has partially to do with the size of the contact patch, but also the load capacity that varies with air pressure, and also how evenly spread the load is around/across the contact patch. The only way to know is to test it.

But with very high speeds, another challenge becomes heat buildup from the energy being reacted through the flexing carcass. So it makes sense that at very high sustained speeds, a higher inflation pressure will keep the tire from going boom. As a related side note, in Formula 1 racing this year Pirelli (who makes their spec tires) implemented a rule that sets a minimum cold pressure for their tires. This was because some tires flew apart during races for no apparent reason other than heat and insufficient load capacity. The teams are pissed because the tires have less grip and a very narrow sweet spot for running in the proper heat range, but at least Pirelli doesn't have to explain exploding tires. So yes, there can be a tradeoff between the pressures for best heat tolerance and best grip.

 

Great post you made, and this quote was especially interesting; it was Discount Tire that told me the C5 Z06 tires didn't have the load rating that the C4 requires. I didn't believe them and argued the points...but they didn't listen. So, I brought them just the wheels.

 

This is exactly the reason why it's always recommended to check tire pressure to make sure it's not low. You don't want to develop excessive heat on extended trips on the highway.

Along the DiscountTireDirect comment where they avoid selling a tire/wheel combo not designed for a given car, this isn't uncommon. Back in the early 2000's, Nitto made a non-Z rated 275 40 17 that was "all-season". I had to remove my wheels and have the tires mounted off the vehicle. Basically, corporate policy is avoid selling a tire with a lower speed rating than the car is capable of -- because they can't know how the car is used.

I always thought the higher pressure ratings in the door jamb were to prevent blow-out during extended use at high speed. Of course, we don't KNOW what speed they had in mind.

After running those Nittos I talked about (which I hated), I ended up going back to a Z tire. I also went to 11" rims with 315 rears -- due to grip dissatisfaction. (Of course the original Gatorbacks were still on my 89 car in 1999 AND lower speed rated tire categories have firmer rubber compounds to achieve higher mileage ratings.) In short, I might not have given 275's a chance because of my tire age/selection. That said, the 315's are probably the "right size" because I converted to a 383 which added more grip challenge.

In riding the fence between firmness (30-32PSI) versus ride (26-28PSI) for the past 10 years, I've found advantages to each....also frustration.

Even in the 28-30PSI range, my first set of Sumitomo 315's wore more in the middle than the edge. I thought this odd. Now, I'm on my 3rd set -- which seem to be wearing more evenly -- even after letting pressure slip near 20 last winter. I'm at a loss to figure out why the middle wore faster on that first set.

I've never been happy with 70+MPH balance. I even had tires road force balanced. I have almost always been able to note vibration above 65-70. I finally decided local road/hwy condition was part of the issue.

At the same time as my 3rd set of Sumitomos were mounted, I installed new wheels, ball-joints, bearings, bushings, tie-rod ends, rear toe-bars, and (later) heim rear dog-bones. Finally, after over 12 years of ownership, the vibration seemed gone. When I did feel it, I confirmed road condition cause it (by retracing my path and observing WHERE the vibration occurred).

That said, it's coming back. I guess it's time to check balance again...though mileage probably isn't more than 5k-10k on the tires. It's still intermittent -- but there. My point here is that lower pressure reveals imbalance more. If I run my partially worn tires at 26 PSI, I can feel more vibration than at 32PSI. At 32 PSI, I think most/all of the vibration is road induced. At 26PSI, the mammoth width of the 315's can't entirely be overcome.

I'm saying wider tires can't stay perfectly round/balanced for long. Compared to thinner tires, you may run into dissatisfaction with hwy vibration by choosing the lower-pressure option. And, once worn, you may not be able to rid yourself of vibration (due to inherently uneven wear of any tire). Knowing this will help you decide what pressure might be your preference.

From my personal observation (over 17 years of ownership), I feel an aversion to running above 30 PSI. When I bought the car, the PO had the Gatorbacks blown up to 45PSI which was the max rating stated ON THE TIRES. As noted by another owner, it's just WRONG to run max on the side of the tire -- without looking what the car is designed for.

I think lower pressure is fine but you have to check it more often. It's easy to dip down to 20 when you start at 26. I think it's also easier to wear tires unevenly at lower pressure. (This is mostly a guess based on personal observation + knowing about heat buildup in lower pressure tires).

With 275's on the front and 315's on the rear (of a standard base suspension) 89, I really think 28-30PSI provides the best all-around comfort vs performance. Above 30 is better when driving hard. Below 28 runs into the problems I talked about regarding longterm wear...long with perception of vibration problems as the tires wear.

I'm sure this may be different depending on roads in your area. I think overall quality of roads have gone down in conjunction with governmental funding problems. This may also play a factor.


PS. I should add that I only have 80k on my 27 yr old car. It was age that made me decide it time to redo the suspension.

 

Also measure your tire pressure AFTER you have driven around a while, it will increase with heat.

I have always had rear tire center wear, especially on 315s, and I have tried various pressures, from high 20s to high 30s. I usually run the rears around 32, and fronts around 35, for better rear grip/ride, and front steering response. Remember there are other factors too, breaking, MPG, and with larger wheels and low profile tires, the low pressure may make the wheel more prone to bending/cracking. I run my C5Z with HRE's w 35 all around to prevent wheel damage.

On my Passat the low pressure light will come on if it goes under 32.

 

True. Most of the time when people quote pressures they are referring to cold pressures, though. My BFG Rival S tires probably gain 8-10psi over four autocross runs, but I bleed them back down to 30psi between runs.

Most likely center wear will be an issue on a 315 unless it's on at least an 11" wheel. Narrower than that and you are sucking in the sidewalls at the beads. You can run lower pressures to avoid center wear, but the tires will be more sensitive to just the right pressures.

 

I ran them on 11", but still always more center wear. Wouldn't there be more growth in the center on a wide tire from centrifugal force, than on a narrow tire, and vary by make. It seems logical to me, but I would think some very well made tires, with a lot of stiff belts, may not flex enough across the tire for the center to be higher.

 

This is what I was talking about earlier. I've experienced this on:
*'80's F-bods where I ran the "legendary" 245/50-16 tires on stock, 16" rims (GTA or IROC) (right sized tired for the rim)
*My stock '06 C6 which IIRC ran a 285/35-18 on the stock rim (right sized tire for the rim
*Now on my '92 with '04 Z06 wheels I'm running the 295/35-18 which is the stock/right sized tire for the rim (C5 Z06).

All of the above was with a variety of tires/brands over many years -even decades.

 

A little late, but great thread!

I am running Continental Extreme Contact DW 285/40/17's on all four corners and I just reduced the pressure from 35 to 28. Quite a difference, car is much less darty as well!

 

 

I noticed the same center wear issue as Gregg with the Sumitomo's. Even when I ran 275/40 on the stock rear wheel. (Stock size was 285). I think this is a Sumitomo construction issue, and probably why they are among the least expensive.

I think that inflation pressure on wide, low profile tires has everything to do with sidewall construction. The sidewall is what decides speed rating and load capacity. Too little pressure is going to compromise sidewall support. This is part of the reason you want a higher pressure for performance driving. Of course, in straight line racing, the lower pressure in rear tires is an advantage, but then there is no real cornering stress, and the run is short enough that heat is not really the issue. But when talking road course or autocross, it's an entirely different ball game. Sidewall flex is a dangerous enemy as well as heat. A firm sidewall in cornering will help to maintain a better contact patch on the track. Also keep in mind that there is greater down force pushing on the car at higher speeds.

My '96 called for 32 psi for normal driving and 35 psi for performance driving. Lowering the tire pressure for a smoother ride is always an option, so long as the driving isn't 'spirited' for any length of time. Remember sidewall integrity. The C4 was always known for it's stiff ride, especially the Z51 cars. Personally, if it's a softer ride I'm looking for, I'm going to look at shocks before reducing tire pressure below recommendation.

 

But you were running 315's on the stock 9.5" wheel. Pulling the sidewalls inward is going to change the profile. I'm running 315's on 11" wheels.

THAT SAID....This newest set SEEMS to be wearing a LITTLE more evenly though I noticed something today...

I have a new 315 Sumi in my basement. In looking at the profile, it's rounded on the shoulders. IOW, it's molded lower on the outside than the inside. Even when mounted and inflated, this may contribute to faster wear in the middle.

I have run lower pressures on two sets in the past....averaging in the mid 20's when you consider leakage/shrinkage. Around town, I would declare mid 20's pressure safe for the sidewalls. But, I never drive mine out-of-town (on the hwy) where tires could develop more heat.

 

Yes, I did the 315 experiment once. The next set were 275's as they don't make a 285. The treadwidth on the Sumi's seems to be a little narrower than some other brands. The 315's especially, compared to say Goodyears are noticeably narrower. You observation about the 'shoulder', or lack there of on the Sumitomo is correct.

I imagine that just normal driving around town at lower pressures isn't going to cause too much havoc. However, it is that psi that attaches the tire to the wheel, and with that torque monster you have, I'd be concerned about that wheel starting to spin faster than the tire...lol

 

Well played, Chris!

 

Pulled my car out of storage this spring. Looked at the rear tires....check 'em out:




....Worn in the middle.


.

 

91 I've always run 30 R 32 F.

 

^That's what I'm taking about. They're calling for 35 lbs on a 315 tire that has~750 lbs on it? DUMB. That's way too high.

 

..... 14.5-15.0 psi in the M/T stiff sidewall 10" wide slicks … 45psi in the front skinnies … Ride Quality is freakin' awesome for the 1st 1/8th mile then I have to slow down and do a 180 off the track and onto the return road … one of the roughest return roads in Florida … Took the vette out for several short drives @ test n tune tonight and it rode fantastic ! … fuel economy absolutely sucks ! ….. Zombie Thread BTW …..

 

I have brand new tires on my vette, using C5 wheels.
I just lowered the pressure to 28, i had issues with the center wearing out on the old rears.

My ride is so harsh with the 84 Z51 package, I'll take what i can get at this point!

Also del-a-lum and banski rods helped with the harsh ride quite a bit...

 

Wasn't GM's increasing recommended tire pressure 32->35 psi at least a little about CAFE rating? My 96 recommends 35 all around, and I never run my rears over 30. One time I was driving my car kind of hard on a back road and I thought something was really fudged with my ride/suspension. Oil change place had inflated my tires to 35, per placard. Heated up, tires were nearly 40 psi. Horrible ride.