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

By Andy Hollis
Jan 6, 2024 | Nankang, Wheels & Tires, CR-S, Nankang CR-S | Posted in Tires & Wheels , Features | From the Oct. 2023 issue | Never miss an article

Photography by Andy Hollis

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The internet is full of motorsports keyboard warriors who will debate endlessly about the theoretical superiority of their favorite components or setup. Specs and physics equations are wielded like swords on the battlefield of paper builds. 

Done well, this approach can yield some useful information. After all, simulation works on the same principles and has proved its value. But at the end of the day, back-to-back testing is the gold standard, especially when it comes to tires.

When testing, isolating variables is key to getting good data and deriving appropriate conclusions. Still, confirmation bias can creep into the mix, as sometimes we get results that we just don’t quite believe. 

What to do? Repeat the test, looking out for uncontrolled variables. Driver consistency, track evolution, ambient temps, prior heat cycles, order of running and even production batches can have an impact.

Two recent GRM tire tests left us very excited, as the combination of their results produced the fastest lap times our Triple Threat ND2-chassis Miata had ever turned at Harris Hill Raceway. One was the introduction of the 2023 revision of the Nankang Sportnex CR-S, the new 200tw top dog for track use. The other was a long-term case study on the effects of heat cycling and tread depth. 

[Tested: How does the new Nankang Sportnex CR-S compare?]

Street tires typically get quicker and more consistent as they’re used on track, with heat cycles curing the rubber and the lower tread depth better controlling squirm and heat buildup. Less tread also makes for a lighter tire and shorter gearing, both of which help in acceleration.

As Nankang ramped up production of the new tire, we saw two new sizes that could possibly perform even better than the 245/40R17 we’d been running on our 17x9-inch Flyin’ Miata wheels. The first was a 235/40R17, which provides a smaller overall diameter and lighter weight, similar to a well-worn (or shaved) 245. Theoretically, that should help it accelerate quicker–and the Miata needs all the help it can get in that department. 

The other new size that looked enticing was a 245/40R15, which would deliver an even shorter, lighter tire. If some is good, more is better, right?

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


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.

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Colin Wood
Colin Wood Associate Editor
7/31/23 9:50 a.m.

Looks like I have a new book to add to my reading list.

kjs9471 GRM+ Memberand New Reader
7/31/23 10:31 a.m.

Finally! Thank you for the info!

7/31/23 12:38 p.m.

Well I was holding out for that 235 to pop up on TR, but perhaps I'll just pull the trigger on the 245. Thanks, Andy! 

racerfink UberDork
7/31/23 12:57 p.m.

Hey, I recognize that track in the photo!  Lot greener looking than when I'm usually there though...

Tom1200 PowerDork
7/31/23 1:07 p.m.

As always this stuff varies from car to car and very likely driver to driver for said car.

I bought Paul Haney's book a few years back; I found it to be handy.

Andy Hollis
Andy Hollis
8/1/23 6:50 a.m.
Tom1200 said:

As always this stuff varies from car to car and very likely driver to driver for said car


Our testing is intended to be a single data point, useful to weed out the low performers and investigate ideas.  Clearly, optimizing the car setup (and driver) for each tire would be ideal...but also logistically impractical.  So we do what we do.  Which also mirrors what most competitors will do (slap on a different set of tires and go).  Only the most resourceful and sophisticated will go to great lengths to do comprehensive personalized testing.  Even then, our work at least gives a starting point, if not a final answer.

camopaint0707 Reader
8/1/23 8:26 a.m.

what was the wheel used?

codrus (Forum Supporter)
codrus (Forum Supporter) GRM+ Memberand PowerDork
8/1/23 10:07 a.m.

Unfortunately the Haney book is out of print, there's no e-book version, and used copies are going for crazy prices online.  I borrowed a copy from a friend to read and it had a lot of interesting things to say, so it's definitely something to keep your eyes open for.

I am curious about the expectation that shorter gearing would always be better?  Isn't that a very course-dependent thing?  Is it just that this is a track and car that you have a lot of experience with and know that shorter gearing is beneficial here?


Tom1200 PowerDork
8/1/23 10:46 a.m.

In reply to Andy Hollis :

I should elaborate as my comments were not aimed at the testing procedure.

Some drivers will not use the extra lateral grip provided by the wider tire; I know some solidly competent drivers who won't push the car to the ragged edge in corners, they'll run about 95% of the car's capability and so a lighter tire may net them a slightly faster lap time do to the lower rolling resistance.

Some cars need all the help they can get; I run Hoosier TDs on my Datsun 1200, between the lower gearing, rolling resistance and lighter tire it makes a difference. Note the car is very underpowered, if the car had 30-40 more horsepower I'd reconsider this set up.

Tom1200 PowerDork
8/1/23 10:52 a.m.

In reply to codrus (Forum Supporter) :

In the Datsun the shorter tire / gearing benefit is two fold; it runs the motor higher up in the RPM range (where the power is) in any given corner and it allows me stay in 3rd and 4th thereby negating 4-6 extra shifts per lap. 

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