Tested: Does a shorter, lighter tire always equal more speed?

Nankang Sportnex CR-S

We still had a set of the 245/40R17 tires mounted up on our Flyin’ Miata Kogeki alloys from that previous tread depth story, and they sported minimal wear. The other two fresh sets were each mounted up and given our standard heat cycle prep: a 45-minute drive to the track, six laps of increasing intensity, and then the drive home. After cooling for 24 hours, our new tires were ready.

Summer is not our favorite season to test, as ambient temps rise quickly in the morning, and the afternoons are simply too hot. Fortunately, test day dawned cloudy, which helped moderate the temperature swing of the track surface–though ambients were already in the mid-80s. We did a session on the 245/40R17 tires to warm up and clean the track surface, then took a break to allow the car and tires to cool. 

With the lap timer now running, we went back out on those same tires to set a baseline. While feeling familiar, they didn’t seem quite as consistent as they did during our winter testing–back when the temps were 40 degrees cooler. Still, all six laps were within six-tenths, with a best of 1:26.7.

Moving to the 235/40R17, we found a noticeably more elastic and skreechier-sounding tire. And while all the laps were within a similar spread as the baseline passes, the quickest–a 1:26.9–came earlier in the session. The tire seemed to be heat soaking fairly quickly. Adding to the mix, the clouds had now disappeared, allowing the sun to quickly warm the track surface.

For the next round, the 245/40R15–fitted on 15x9-inch Kogeki wheels–delivered the same feel and sounds as the 235s but recorded an even slower pace with a best lap of 1:27.4. On the plus side, they felt more nimble and responsive, but they couldn’t perform as well when asked to combine loads during trail-braking and corner exit power-up. The latter counteracted any gearing benefit on acceleration from the shorter tire.

While most 15-inch wheels will not clear the brakes on an ND-chassis Miata–especially with the Brembo option–the Kogeki line from Flyin’ Miata will. 

We always bracket our tests by going back to the original tire to see if we had properly controlled variables. With temps now 10 degrees warmer and the track surface getting very hot from the sun, we expected slower times. Instead, the 245/40R17 tires cranked out three consistent laps–all a couple tenths faster than before and with a fastest of 1:26.4. Shocked, we went to the data logger for an explanation: The improvements came in two areas where both driver confidence and a cleaner track provided a benefit.

Faced with the specter of flawed data from track evolution and driver improvement, we kept the test rotation going by again mounting up the 235/40R17s. They also picked up time over their previous iteration with a best lap of 1:26.6, but they remained the same two-tenths slower than the 245s in the back-to-back sessions. This confirmed our comparison of those two tires. 

Finally, we ran the 15s again, finding no improvements there at all, with a best lap of 1:27.5. While sounding good on paper, the shorter, lighter tire was simply not the right answer once the rubber met the road.

Test Day 1

All the way home, one thing was bugging us: the distinct difference in feel and audible feedback of the new tires versus the set that we carried over from previous testing. They almost seemed like a different tire. We even contacted Nankang to verify that nothing had changed in the tire’s production. 

Looking at our records, we found that while our 245/40R17s were still at almost-new tread depth, they had a few more heat cycles on them. Was that the cause? Only one way to find out.

Over the next several days, we made two more trips to the track to add to the 235s’ lap count. Midway through that process, the screechiness went away and the tire became more consistent, like the older 245s. Having now removed that variable, we went back for another back-to-back test. We also switched the running order to make sure it wasn’t a contributing factor. 

Test Day 2

Conclusion

So what did we learn? In every back-to-back comparison, the 245/40R17 ran a few tenths quicker than the 235/40R17. Drilling down into the data for more clarity, we found that the 245/40R17 tire performed better under trail-braking and corner-exit acceleration, meaning it could simply multitask better–without giving up anything anywhere else. (Like the 235/40R17, the 15-inch tire also didn’t multitask as well as the 245/40R17.)

To some degree, this result flies in the face of other tire- and wheel-width tests we’ve done. For street tires used in motorsports, we’ve previously found that there is no benefit to running a tire that has tread width any wider than the wheel width. With the new Nankang, the 235 fits the rim width better than the 245, the former having a slight stretch and the latter being more square. Yet the 245 was consistently quicker.

So we dug out our copy of Paul Haney’s high-performance tire book and found an explanation. Just as a wider wheel-and-tire combination shapes the contact patch to benefit lateral grip, a larger diameter shapes it to support grip in the longitudinal direction. Whether this outweighs the benefits of shorter gearing, lower weight and lower cg is very dependent on tire model and vehicle application. In this particular case, though, it did.

 

Paul Haney’s book “The Racing & High-Performance Tire” is a fantastic resource for understanding the black art of motorsports tires. We reference it often. While long out of print, you can find used copies online.

This is why we test empirically rather than using internet wisdom or theories. The latter are good starting points for ideas, but ultimately we’re better off with a real-life comparison. In the end, we didn’t find any new speed, but we did have fun trying–and we learned a few lessons along the way.