C5 Corvette Tires - The Run-Flat Alternative

Admittedly, it's far from a statistically valid survey, but if I were to judge by the letters and e-mails that come into VETTE Command Central, and the comments and questions I hear at various Corvette events around the country, there are a lot of C5 owners who find the factory-installed Goodyear Eagle EMTs to be, well, less than the greatest. Hey, it's no big deal to switch over to some other brand when the OEM run-flats are due for replacement, right? Only it is a big deal, and a wee bit more complex than that.

Actually, there are a couple of complications. First is the fact that the C5 was engineered specifically to use a run-flat or extended mobility tire, and there is no provision in the C5 Corvette for a spare tire. Obviously, that's a potential problem, especially if you take that C5 for a long trip. The second problem has to do with the sizes of the tires (and wheels) that The General fits on each and every C5. The front tire is a 245/45ZR-17, a very common size and one that nearly every tire manufacturer offers in at least one of their ultra-high-performance tire lines. The rear tires, size 275/40ZR-18, are real oddballs. That specific size is, as far as I have been able to determine, unique to the C5, with no other OEM applications.

It takes a major (well into the six-figure range) investment for a tire manufacturer to engineer and tool-up molds for a specific tire size, and there has to be the potential for substantial aftermarket sales to justify the up-front investment. It's different for the OE (Original Equipment) supplier. In the C5's case, Goodyear is contracted to construct and supply a given quantity of tires over a specified time period directly to GM's Bowling Green plant, and every C5 rolls off the line on the same spec tires. Unfortunately, there is no impetus for Goodyear to improve on the product, at least as long as Chevrolet is satisfied with the tires.

That doesn't mean there is anything wrong with the OE Goodyear run-flats. But, it also doesn't mean that all of the 74,000-plus buyers of '97-99 C5s are necessarily ecstatic with the performance characteristics and wear of them. And, until recently, there haven't been many alternatives for C5 owners who choose to retain the stock tire and wheel sizes, and either want to upgrade their rolling stock or need to replace worn tires.

But, by the time the 2000 model run has ended, there will have been over 100,000 C5s built and sold. Over 100,000 cars equals over 400,000 tires, and that's a sizable enough market for replacements to get the marketing mavens in many tire manufacturers salivating-particularly when you're talking about high-end, ultra-high-performance specialty tires.

One of the first tire manufacturers to take advantage of this potential opportunity is one of America's oldest tire manufacturers, Firestone, now a part of the Bridgestone corporate empire. Firestone's original Firehawk SZ50 series of ultra-performance tires, which was introduced in 1996, has been justifiably regarded as one of the best tires of its type. This last February, Firestone introduced a second generation of this tire line, called the SZ50 EP (for Extended Performance) at a series of special test sessions for select media and Firestone dealers, held at Firebird Raceway in Chandler (suburban Phoenix), Arizona.

So what's any of that have to do with C5s? A lot! In addition to the usual array of sizes (including ones to fit any C4 except the rear of a ZR-1), Firestone also debuted two new sizes in the SZ50 lineup, with what they're calling "Run Flat Technology"-specifically for C5 Corvettes. That's right, 245/45-17s and 275/40-18s, the correct factory sizes. And just to make things a little more interesting, they had six new 2000 Chevy Corvette convertibles, shod with the new 'Stones, on hand for us to test-on track.

Actually, Firestone was debuting four new tire lines; the aforementioned SZ50 EPs and C5-specific SZ50 EP RFTs, and a couple of new product lines for passenger cars and minivans. There were different diving exercises, at different sites on the Firebird facility, and different types of vehicles to be tested for each new tire. All driving exercises were done with and under the supervision of professional high performance driving instructors. I'm not into minivans or six-cylinder Mustangs, and don't even own a car that would use "everyday" type tires. Evaluating all four new tire lines was part of the program; I did it, the new tires for sedans and minivans are exemplary for their intended purposes and markets, and I'll let it go at that.

The new SZ50 EP is available in several sizes that are applicable for various C4s, so this segment of the program was certainly of interest. The original SZ50 used a lot of the technology Firestone developed for their all-conquering IndyCar race tires, specifically the wet/rain version. The new SZ50 EP is a further development on that theme, with a slightly refined tread design and a new dual-tread compound that reduces hardening of the tread and is claimed to give consistent grip, even as the tire wears. The test/demonstration for this was telling. There were six new Z28 Camaros, and a tight (but fast) wet slalom course. Two of the Z28s were fitted with half-worn Goodyear Eagle F1s (the original equipment tire), the second pair with half-worn SZ50 EPs, and the final two with brand-new SZ50 EPs. The program? Go out and run laps on the Goodyears, followed immediately by the worn Firestones, and then the new Firestones.

The cars were clearly marked as to which tires they were wearing, so it wasn't a "blind" test; pure and simple, drive all three and decide for yourself how they stacked up. Needless to say, the fresh Firestones were the most controllable and the fastest. I was surprised by how good the half-depth Firestones performed, and how close in performance they were to the fresh set. And I was surprised by how much less control was available, and how much slower the Eagle F1s felt. This was strictly seat of the pants, but with the years I've spent driving high-performance cars (and the occasional race car) on cone courses (slaloms) and race tracks, I figure that I've got a pretty well educated seat. If I owned a C4-other than a ZR-1-that needed new rubber, I'd give these new Firestones some serious consideration.

But, it was the new C5 run-flats that I was most interested in trying out. Ever since my first driving time in a C5, I felt that the cars' handling, as well as the intangibles of feel and response, were hindered by the tires. I've also felt that even the Z51 suspension was slightly underdamped and could used heftier anti-roll bars, but that's another story, entirely. Not that there's anything wrong with the stock tires. It's just that, at least to me, they were too compromised. You know, gotta handle acceptably, wear acceptably, provide a good ride, not be too noisy, etc. etc. A lot of times, the OE tires are cheapest ones that will meet the car manufacturer's requirements. One thing the Goodyear EMTs are not is cheap. And they are good tires; Z-speed rated, predictable; it's just that to me they could be better-and that would make an already excellent car better.

The half-dozen 2000 C5 convertibles were all base-suspension automatics. A fairly large (roughly 3/4-mile) loop segment of the Firebird track was set aside for the Vette tests, with a slalom (eight cones, spaced 100 feet apart down the center of a straight stretch) to be woven through, chicanes for back and forth, hard right and hard left turns, plus right- and lefthand sweepers. The routine here was to go out and run five or so circuits-at a very modest 55 mph maximum, come in and observe while a Firestone technician pulled the valve from the right front tire to completely deflate it, then go out and drive the course several more times. At the modest speeds we were limited to, the effects of the deflated run-flat were relatively unnoticeable, except for a hideous grinding sort of noise from the airless tire in lefthand turns (when weight was transferred onto the deflated tire), and gross understeer in the left turn gates of the chicane. It would've been very interesting to have had one C5 on hand equipped with the Goodyear EMTs, for a direct comparison.

I'd made arrangements for a late departure and to have a private, "after hours" track session in one of the Firestone-shod C5s, and that's when the real fun occurred. Professional race driver Peter Cunningham and I belted ourselves into one of the Vettes, headed onto the course, and said, "To hell with the rules, let's see what this thing can really do!" Lap after hot lap, with Peter's coaching and advice, I kept pushing the car's-and my own-limits. By the time we'd finished, I'd wheeled the C5 through the slalom cones, where the "rules" were 55 mph maximum, at 99 mph, and wrung it out comparably around the rest of the layout. Peter took a turn and wove us through the cones at 102 mph!

Without a direct comparison and some testing equipment, seat-of-the-pants impressions are impossible to quantify. However, my impression of the new Firestones is that they felt more controllable, more responsive to input, and grippier than the original equipment Goodyears on any of the C5s I've driven, including the Bragg Smith school cars. If I owned a C5 and was in the market for stock replacement size tires, I'd give the new Firestones some real serious consideration.