Tire diameter more important than width?

[ShifterX]

Looking for some more information on track wheels and tire setups. There seems to be a lot of focus on width and the universal advice of "fit the widest tire you can" being ubiquitous. That's simple enough to understand, however looking at the range of setups it seems clear the S197 mustang can accommodate a wide range of tire diameters. More so than many other cars. Considering width primarily impacts only the contact patch and the diameter affects the contact patch, plus final gearing plus the CG, it seems the overall diameter is the more critical factor.

My 2011 GT/CS came stock with 245/45/19 which is 27.7". Almost a 28" tire seems comically large coming from SN95 cars. Leaving that aside, typically with most near stock builds you would try to target close to the OEM diameter. Is this the case for the S197? It seems many builds end up targeting a much smaller diameter. Here is the Moto IQ build for example.



Billy Johnson ends up using tires almost an inch shorter than stock. That should shrink the contact patch relative to a similar width tire of OEM height. So less rubber, lower gearing and lower CG.

Is there an ideal tire diameter for a track tuned S197? Is it always smaller than stock? My application is similar to Billy's, which is a street car to be used on the track (HPDE some AutoX). Thanks

 

[JDee]

Width reigns supreme. You need all the contact patch you can get and diameter has little to do with that.

 

[Mad Hatter]

Would agree that width rules, but diameter also helps the contact patch but to a much lesser degree as the taller tire makes the patch wider parallel to the tire. Using the ubiquitous 315/30-18 Hoosiers you get a diameter of 25.6" But a 300/350-18 gives you 26.8" . With a 3.73 rear end and 500rwhp I like the taller tire from a final drive point of view.

 

[ShifterX]

Width reigns supreme. You need all the contact patch you can get and diameter has little to do with that.

Tend to agree, but its a simpler calculation. Either class or physical space is limiting width. Those limitations aside, wider is better. Pretty simple. Choosing diameter has more considerations. Going larger gains you contact patch, taller gearing, more weight, higher CG etc. Going lower nets lower gearing, CG, weight and trades out some traction. I see diameters from 25.5 up to near 28.

What diam do you target and why?

 

[SMGDHG]

I'm not smart enough to to understand how an increased wheel diameter would lead to an increased contact patch, but I am smart enough to know that 19" tires are in many cases much, much more expensive than 18" tires. 19" tires often times weigh more, as do 19" wheels, and depending on what tire compound you wish to run, have less size selection. Gearing advantages or disadvantages will depend on your personal car and what tracks you run. If you find yourself running out of gear and needing to shift more with the 18"s, that's a bummer. But also if you find your car is sluggish with the longer gearing of the 19's, that is also a bummer. This will depend on you, the tracks you run, and the cornering speeds of your car.

There are certain tire brands and compounds which have better size options in 19", and there are some which have better size options in 18". I would chose which tire you'd like to run first and go from there. But again, I see little to no advantage to running a 19". If there is an advantage, I firmly believe it is not worth the additional cost.

One more thing I remembered. Depending on your brake caliper setup, some 18's may not fit. All apex 18" wheels fit just about every brake setup imaginable for the S-197, so I would definitely recommend going with them.

FWIW, the majority of the items on Billy's article are just an echo chamber of what the TMO community has been doing for years, so it's overall sound advice but is nothing new.

 

[stevbd]

Agree with @SMGDHG. The best advice I got was to pick the tire first and go from there. There tend to be more 100-200tw "extreme performance" options available in 19" and more true race tires in 18". It seems the 18" offerings are improving somewhat but I think that still mostly holds true.

Oftentimes there isn't a big difference in price or diameter. Go on TR and look at some of the most common options, a 295/35/18 has an OD of ~26.2", a 315/30/18 is ~25.6", and a 305/30/19 is ~26.3". And costs are similar 18 vs. 19.

 

[ShifterX]

Its weird that overall diameter is such an after thought. Coming from lower power better handling cars (RX8/S2000), the idea of a diameter range of 2" is pretty crazy. Those cars just wont tolerate massive tire size swings. Its probably that those cars were shipped from the factory closer to their potential.

Is there a reason it seems that most people end up with track tire an inch or more smaller than what was shipped from the factory. Is it just a tire availability thing, or is there an intended performance gain by going to a 25.5, 26 or 26.5 diameter.

 

[SMGDHG]

Its weird that overall diameter is such an after thought. Coming from lower power better handling cars (RX8/S2000), the idea of a diameter range of 2" is pretty crazy. Those cars just wont tolerate massive tire size swings. Its probably that those cars were shipped from the factory closer to their potential.

Is there a reason it seems that most people end up with track tire an inch or more smaller than what was shipped from the factory. Is it just a tire availability thing, or is there an intended performance gain by going to a 25.5, 26 or 26.5 diameter.

You say those cars wont tolerate massive size swings....exactly what was intolerable, and exactly what changed when you went to the different diameters?

Yes, there are reasons, all of which were stated above: Lower running costs for tire replacement. Lower initial purchase cost for the wheels. Lighter weight. Brake clearance is a non issue if you get the right wheels. As discussed earlier, you get an effective gearing change when you change the diameter . This is a non issue depending on what tracks you run at. It can also be a PRO if you have a car with longer gears like many mustangs which are hilariously long from the factory (hitting 77mph in 2nd gear and nearly 110mph in 3rd gear is useless on a road course, ask me how I know). A smaller diameter tire represents a massive advantage here, and is astronomically cheaper than swapping out rear end gears.

Also please keep in mind that comparing a GT/CS mustang (a glorified rental car) to an S-2000 is very much an apples to oranges comparison when you look at the intended design scope of the two different cars.

 

[ShifterX]

You say those cars wont tolerate massive size swings....exactly what was intolerable, and exactly what changed when you went to the different diameters?

Yes, there are reasons, all of which were stated above: Lower running costs for tire replacement. Lower initial purchase cost for the wheels. Lighter weight. Brake clearance is a non issue if you get the right wheels. As discussed earlier, you get an effective gearing change when you change the diameter . This is a non issue depending on what tracks you run at. It can also be a PRO if you have a car with longer gears like many mustangs which are hilariously long from the factory (hitting 77mph in 2nd gear and nearly 110mph in 3rd gear is useless on a road course, ask me how I know). A smaller diameter tire represents a massive advantage here, and is astronomically cheaper than swapping out rear end gears.

Also please keep in mind that comparing a GT/CS mustang (a glorified rental car) to an S-2000 is very much an apples to oranges comparison when you look at the intended design scope of the two different cars.

You would just never use a 2” smaller or larger diameter tire on those cars. Not saying someone hasn’t done it but it’s not a common route. They just can’t pull the extra mass of the larger tire or sacrifice contact patch for something smaller. There is just a smaller window for what fits and works. I think you hit the nail on the head with the apples to oranges comparison.

Back to the glorified rental car. I can appreciate the benefits of a 18” wheel. However I was not even considering wheel size, initially. Just the overall diameter. Now I see some of the issue. To drop to an 18” wheel and still keep the oem diam you would need a 40 series tire (assuming a 305). A quick check reveals nothing in a 305/40/18. So going to an 18” wheel is akin to going to a smaller diam tire.

There are just less performance tire options trying to stay with the oem diameter.

thank you,

 

[Bill Pemberton]

This is a conversation that could go on forever and likely get more technical with each thread when sometimes the easiest answer is what works with the fast TMO Trackrats and then it becomes much simpler. JDee mentioned width reigns supreme and he is so dang old you need to listen to that Old Canuck, but there are a ton of us old farts on here that learned the hard way too. Old fart suggestion is go with a 18x11 wheel, slap on a 25mm spacer, and get some fat, sticky 18s.

18s are cheaper, still have more options , and are used by a boatload of folks here. Plus they also give you a better chance of securing used race tires if you get to that point in the next few years. Honestly you had all the info just seeing what Billy Johnson uses! Fat is where it is at, at least with tires !

 

[Mad Hatter]

All true!!! Dont forget! once the car is lowered and set up for racing.... a tall tire will probably not fit very well!!!

 

[xr7]

I run CAM C in autocross, with a square set up, Falken 295/40-18's on Apex 18 x 11's. This tire is slightly taller, 27.4" vs 27.2, than the OEM tires. These tires really fill the wheel wells. If I was to go any wider at this height I'd have to get fender flares front and rear. I'm probably going to move up to 315/30-18's for next year. The side effects are going to be more shifting or changing the rear gear. I have 3.73 rear gears and the stock 444 hp is more then capable of boiling the tires so the shorter tire gearing advantage isn't what I need. Hopefully a little more contact patch and shorter sidewalls will get me a little more speed thru the corners. Six gear changes in an autocross run vs one or two is more challenging but also brings a big smile when it results in a fast run.

 

[blacksheep-1]

Diameter is important, it's why the dragster guys have gone from 14 in wheels to 16, and maybe more over the years. When the tire rotates around, the tire flattens out on the bottom so the patch is bigger on a bigger diameter tire. However.. and this is a big however, of the, maybe 4 components of a tire, diameter would be the last. Certainly compound, width and sidewall would always be more important than diameter, but to function properly they all have to work together. This is one reason that I'm sweating going from a 68 to a 65 tire on the front of the new mustang. The new car also has a narrower front width, and of course, is a strut car, not an A arm car, which exacerbates the problem.

 

[EF1]

Looking for some more information on track wheels and tire setups. There seems to be a lot of focus on width and the universal advice of "fit the widest tire you can" being ubiquitous. That's simple enough to understand, however looking at the range of setups it seems clear the S197 mustang can accommodate a wide range of tire diameters. More so than many other cars. Considering width primarily impacts only the contact patch and the diameter affects the contact patch, plus final gearing plus the CG, it seems the overall diameter is the more critical factor.

My 2011 GT/CS came stock with 245/45/19 which is 27.7". Almost a 28" tire seems comically large coming from SN95 cars. Leaving that aside, typically with most near stock builds you would try to target close to the OEM diameter. Is this the case for the S197? It seems many builds end up targeting a much smaller diameter. Here is the Moto IQ build for example.

View attachment 90632

Billy Johnson ends up using tires almost an inch shorter than stock. That should shrink the contact patch relative to a similar width tire of OEM height. So less rubber, lower gearing and lower CG.

Is there an ideal tire diameter for a track tuned S197? Is it always smaller than stock? My application is similar to Billy's, which is a street car to be used on the track (HPDE some AutoX). Thanks

It looks like you read Billy's budget build article. Did you also read his article on selecting tires/wheels?

How to PROPERLY select and size TIRES for PERFORMANCE - MotoIQ

How to PROPERLY select and size TIRES for PERFORMANCE by Billy Johnson The most important part of your car is not the engine, suspension, or brakes. It’s the TIRES!!! This…

motoiq.com

It goes into more depth on the subject that the budget build series did.

 

[ShifterX]

I did, he dives into the importance of diameter and typically recommends staying with oem diameter, at least to start. It was a good read.

plenty to consider, but as @blacksheep-1 mentioned diameter will be a bit of a function of the other major factors.

With 373 gears, I really don’t need much smaller diameter. Certainly don’t want to go any taller than stock. If I go with 19s anything around high 26 to mid 27 will work.

thanks.

 

[noldevin]

Width is also important because it helps the tire stay cooler, which is valuable on a heavy and high horsepower car. My 305/30/19s are 26" tall. I would prefer a little taller just for gearing reasons, but the fit is already tight and it's just not an option with available sizes.