Tech Corner
The Tire Rack's Experts Answer Your Tire-tech Questions

Imperfect Match?
I just purchased my first Corvette, an '89 convertible with 80,000 miles. My mechanic pointed out to me that the door sticker calls for P275/40ZR17 tires, but the car has P245/45ZR17 Dunlop SP Sport 8000 tires installed on the front and P245/45ZR17 Dunlop SP Sport 9000 tires on the back.

My questions are as follows:
1. How do the different size tires and tread patterns affect the performance of the car and the accuracy of the speedometer?
2. Is it OK to put larger tires on the back than on the front?
3. Is there a manufacturer and tire you would recommend?

Richard Ferguson
Newmarket, Ontario

While both the Dunlop SP Sport 8000 and SP Sport 9000 tires feature directional tread designs, The Tire Rack tests showed the SP Sport 9000 trades some dry-road responsiveness and handling for enhanced hydroplaning resistance, compared with the SP Sport 8000. So even if both your front and rear tires had the same amount of tread depth, the front tires would tend to respond better at the limit in the dry, while the rear tires might better resist hydroplaning in wet conditions. Unfortunately, this could cause the car to handle differently in dry versus wet conditions, and it is one of the reasons that mixing tires with different characteristics isn't recommended.

The 245/45R17-sized replacement tires have approximately the same overall diameter as the OEM P275/40R17 tires and will not alter the accuracy of the speedometer.

Narrower tires tend to be less responsive and a little lighter than wider tires, so it's quite possible your car's previous owner was trying to soften the convertible's ride quality and reduce road noise. Fortunately, the 245/45R17-sized tires offer sufficient load capacity and can be mounted on the factory 9.5-inch-wide wheels.

Putting bigger tires on the back than on the front (staggered sizes) is a common practice for the drag strip, but isn't a great solution for the street unless the car's suspension has been tuned for it. While the Corvette ZR-1 came with P275/40R17 front and P315/35R17 rear tires to help harness its horsepower, Corvette engineers tweaked the ZR-1's suspension to maximize traction and handling with this combination. While simply putting bigger tires on the back will increase traction for accelerating, it will prevent you from rotating your tires and will probably result in more understeer when cornering at the limit.

Worn Out
I'm having trouble with uneven wear in the front tires of my '02 Z06. The inside tread wears substantially faster than the outer tread. I have taken the car to my tire dealer for an alignment check, and he indicates that everything is within Z06 spec. Do you have any experience in this regard?

Bill Brenneke
Via the Internet

I hate to hear an alignment technician report a vehicle is simply "within spec," because it often indicates they didn't do a very thorough job. This is because the vehicle's suspension-alignment specifications provide preferred settings as well as the allowable range above and below those settings. Saying an alignment is "within spec," as opposed to setting the angles to the preferred spec, can be the difference between just going through the motions and doing the job right.

Front-tire wear on a Corvette is typically related to the type of driving, as well as to the camber and toe settings. Your Z06 calls for a front-camber angle of -0.70 degree and allows for up to +/-0.50-degree tolerance. This means that anything from -0.20 degree to -1.20 degrees is within spec. If you drive your Z06 very aggressively, -1.20 degrees of camber may be fine; however, if you drive a bit more sedately, the -0.20 degree may provide more even tire wear. Unfortunately, the technician's report doesn't provide enough information to make this determination.

Additionally, the Z06 calls for individual front toe of +0.04 degree (toe-in), with a range of +/- 0.10 degree per side (total front toe of +0.08 degree, +/- 0.20 degree). Again, this results in a front-toe range where everything from +0.28 degree (toe-in) to -0.12 degree (toe-out) is within spec. Since the wide tires on Z06 Corvettes respond well to near-zero toe, you want to make sure your car doesn't have any more toe-in or toe-out than is absolutely necessary.

Suspension alignments are tunable to blend driving style and vehicle tolerances with tire performance and wear. A good alignment technician should be able to talk with the driver, examine the tire wear, and set the suspension angles accordingly.

The Sound of Silence
I have an '04 convertible, and the factory Goodyears are in need of replacement. I'd like to install something that will cut down on road noise without sacrificing performance. Is this possible, and, if so, what brand and model would you recommend?

Roger Neale
Via the Internet

Based on the tire-satisfaction survey responses we receive from Corvette drivers on www.tirerack.com, one of the tires you will want to consider is Firestone's Firehawk SZ50 EP RFT. This tire was developed in 245/45R17 front and 275/40R18 rear sizes specifically for the C5 Corvette. The reviews are very positive in regard to ride comfort, noise quality, and traction; however, a few drivers have reported wearing out their rear tires in about 15,000 miles.

Another tire you will want to consider is Michelin's Pilot Sport A/S ZP, which was also developed in 245/45R17 and 275/40R18 sizes specifically for the C5. While you may never need the Pilot Sport A/S ZP's all-season light-snow traction, the reviews are again very positive when it comes to ride comfort, noise quality, and traction.

SOURCE
The Tire Rack