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

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 …

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

Finally! Thank you for the info!

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! 

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

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.

Tom1200 said:

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

Indeed.

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.

what was the wheel used?

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?

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.

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. 

codrus (Forum Supporter) said:

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?

It does depend on the track, for sure.  For example, if you are currently rolling right up to the rev limiter on the longest straights, and coming off most of the turns in the meat of the torque curve, then shorter may hurt.  In this test, I had to upshift in one place with the 15's that I did not for either of the 17's, but I also did not downshift/upshift in one transitional section which  I typically have to use 3rd gear for to maximize acceleration.  So it was a wash in that regard. 

But the straights themselves were quicker in accel for both shorter tires.  Then again, as mentioned in the story, I could not apply power as soon with the shorter tires. So the straights also became a trade-off.

camopaint0707 said:

what was the wheel used?

It's in the story.

hmm, if 235/40/17<245/40/17 on 17x9, makes me wonder about my typical go-to of 255/40/17 on 17x9 for my STX / Tuner 4 RX-8... if wider is better and wheel width is restricted by rules...

In reply to Matt Huffman :

In the example from the story, it was true of the CRS.  Doesn't mean it's true for other street tires.  Might be, might not.  Only one way to find out for sure. 

I'm surprised to see the gap being so wide between the 15's vs 17's on a lower powered car like the Miata. I wonder if the premise holds true for even lower powered cars. Thanks for taking the time and writing this up! 

So, theoretically, if you had a tall wheel/tire vs a short wheel/tire that where the same width and weight and the gearing was changed to keep the engine in the same power band, which should be faster? Tall tire with slower rpm or the short tire with higher rpm?

Pretty interesting article.

Has me wanting to go from a 15x9(225s) up front, to a 17x9(235 or 245) up front on my K20 TT5 DC2.

I'm limited at my weight to a 257mm measured tire, so 255s will be a out, unless probably on a 9" wheel (and also depending on the tire), currently running RC1s.

Any input? Think I'd go with a 17x9.5 if that was common in a 4x100, don't want to 5 lug the front yet, since I'm limited to a 257mm anyway.

Blackhalo said:

Pretty interesting article.

Has me wanting to go from a 15x9(225s) up front, to a 17x9(235 or 245) up front on my K20 TT5 DC2.

I'm limited at my weight to a 257mm measured tire, so 255s will be a out, unless probably on a 9" wheel (and also depending on the tire), currently running RC1s.

Any input? Think I'd go with a 17x9.5 if that was common in a 4x100, don't want to 5 lug the front yet, since I'm limited to a 257mm anyway.

I'm curious, what do your 225 RC1's on a 15x9 wheel measure at? 

In reply to roninsoldier83 :

It's close to the 226mm template. But fits. 

My car isn't maxed so I don't need the +.3 moddifier, not sure if I'm even taking it now on my current sheet.

Other option is to go to a 15x9.5 and run a 245 RC1, but leaning 17's. Ideally I'd go 17x9.5 and run a 245, but really don't want to spend the money on hubs, rotor hats etc for 5 lug.

Andy, you do such a tremendous job on these tests.  Everything right and more within the bounds of reality.  Bravo!   

randyracer said:

Andy, you do such a tremendous job on these tests.  Everything right and more within the bounds of reality.  Bravo!   

Thanks for the kind words. 

Some tests come out better than others...this was one of them.

When you do something for 13+ years, you learn a few lessons.  Here's the very first GRM test I did: https://grassrootsmotorsports.com/articles/texas-toasted-tires-part-1/

The amount of info/resources we have on this forum never ceases to amaze me.

Indeed. 

Really appreciate these articles on tires.

This article convinced me to jump to 17s from 15s for my Integra. Even though I could run the same width wheel/tire on either size.

I've read a lot of people say similar to what this test showed, but this confirmed it for me.

Is it also possible that the internal belting angles are different between the different tire sizes?  I know in the DOT race tire world there are some differences that change the ability of the tire to multitask within the same tire model