Stretched Tire Theory.

[Grintch]

He's saying that a 255 on a 9.5" will be faster than a 255 on a 8.5" AND that a 285 on a 9.5" will be faster that a 255 on a 9.5".

No he is not. He is saying that a 10.5" wheel is optimum for a 10" (254mm) tire (which is outside the typical manufactures recommendations of a max 10" wheel for a 255 tire). Then he says a wider tire is better for a even narrower wheel.

If 10 or 10.5 wheel is optimal for a 255, then a bigger 285 tire must be sub optimal (worse). Which is why that made up guideline is junk.

Sponsored

 

[BmacIL]

No he is not. He is saying that a 10.5" wheel is optimum for a 10" (254mm) tire. Then he says a wider tire is better for a even narrower wheel.

If 10 or 10.5 wheel is optimal for a 255, then a bigger 285 tire must be sub optimal (worse). Which is why that made up guideline is junk.

Which 255 tires have 10" tread width? Section width, maybe, but not tread width. Tread width is what he was referring to. Most 275/35R19 tires, for instance have a tread width around 9.6-10.0" (many are 9.8"), despite the section width being ~11". That would lead one to a 10-10.5" wheel, ideally, though they still fit and perform great on a 9.5" wheel.

 

[SteveW]

No he is not. He is saying that a 10.5" wheel is optimum for a 10" (254mm) tire (which is outside the typical manufactures recommendations of a max 10" wheel for a 255 tire). Then he says a wider tire is better for a even narrower wheel.

If 10 or 10.5 wheel is optimal for a 255, then a bigger 285 tire must be sub optimal (worse). Which is why that made up guideline is junk.

I think the disconnect here is I'm assuming everyone knows to go by what the tire tread width spec is and not what is stamped on the sidewall. You must be assuming I mean the size stamped on the sidewall = tread width. I may not have been completely clear on what tires i was referring to either.

My 255 tire on my 10" has a tread width spec of 9.4" and max size of 10" wide wheel. It would not be better (or really safe) to put that tire on a 10.5" wheel. The 285/35/19 RE71R is happy on a 10" wheel but ideally would be on a 10.5" because the tread width spec is 10.1". You can run it on an 11" wheel but I'd only do that if you already have 11" wheels and got the tires for free or something like that. The 305/30 would be better on the 11" wheel.

I have tested 205/50 vs 225/45 on a car I used to autox that had 7" wheels. I don't have the notes but what I remember is no real difference in times between the tires on the hybrid gokart/autox course I was on but the 205 felt better and was easier to drive. However, the car was on the stock (too soft) suspension and the 205s weren't handling the load very well so I used 225s. (Scary looking rollover in pics) Same thing happens to S550 FS guys: 275s and 285s on 9/9.5" wheels are about the same on the clock but 285s hold up better for multi-drivers or hi heat days. Again, this case is for a car on a stock suspension.

 

[Mountain376]

How bout 285 re71r on 11 inch wheel vs 10 inch wheel? How much faster will the 11 inch wheel setup be, very roughly speaking?

To be honest, you likely wont notice any time difference outside of drag racing.

I am saying "likely" because I am being REAListic. In that, I mean I have tried/been associated with trying this type of thing through years of autocross, track and some road racing. The biggest two things you will notice is 1. tire wear and 2. steering/car response.

 

[Grintch]

I think the disconnect here is I'm assuming everyone knows to go by what the tire tread width spec is and not what is stamped on the sidewall. You must be assuming I mean the size stamped on the sidewall = tread width. I may not have been completely clear on what tires i was referring to either.

My 255 tire on my 10" has a tread width spec of 9.4" and max size of 10" wide wheel. It would not be better (or really safe) to put that tire on a 10.5" wheel. The 285/35/19 RE71R is happy on a 10" wheel but ideally would be on a 10.5" because the tread width spec is 10.1". You can run it on an 11" wheel but I'd only do that if you already have 11" wheels and got the tires for free or something like that. The 305/30 would be better on the 11" wheel.

I have tested 205/50 vs 225/45 on a car I used to autox that had 7" wheels. I don't have the notes but what I remember is no real difference in times between the tires on the hybrid gokart/autox course I was on but the 205 felt better and was easier to drive. However, the car was on the stock (too soft) suspension and the 205s weren't handling the load very well so I used 225s. (Scary looking rollover in pics) Same thing happens to S550 FS guys: 275s and 285s on 9/9.5" wheels are about the same on the clock but 285s hold up better for multi-drivers or hi heat days. Again, this case is for a car on a stock suspension.

The issue is that while a 10" wheel probably is optimum for a 255 TIRE. A a 285 or 295 tire is optimum for that WHEEL, and that car.

Now if you have a 130hp, 2500lb car more tire is not the best solution for everything. Though I suspect more because the bigger wheel and tire is heavier that because it has the wrong stretch. And the whole stretch theory completely ignores the wide variation in sidewall stiffness across different tire compounds. I know back when I was heavily into autocrossing, all the stock guys ran 275 R compound tires on dinky 8" wide wheels (a good two sizes beyond the typical tire size recommendation) in my class and won National Championships that way.

Assume you have 10" wheels and a 3900lb car (with driver) with 400+hp. A set of Hoosier R7 in 255 will be faster than any other non R compound tire, 245, 255, 285, 295, 305 doesn't matter.

For a given car, the tire size is the primary limiting selection. So choose it next. For our big powerful car. The biggest you can fit will be fastest (no Miata too big tire issue here).

Now choose the widest, lightest wheel you can afford that hopefully looks good.

Thus a 285 tire on a 10" wheel or 305 on a 11" wheel are prime choices for the V8 Mustang, rather than a narrowish stretched 255 on 10" or 285 on an 11" wheel that the suggested rule of thumb would require. Now if we had more wheel and tire selection and a bit more clearance in certain spots maybe we would all be trying to run another half inch or inch wider wheel. But selection in 10.5 & 11.5 wheels stinks. And 12" isn't great either for the Mustang.

But feel free to run those "optimum" little stretched 255's. Just watch your mirrors so you don't hold me up with my sub optimal 285's.

 

[AlbertD]

The issue is that while a 10" wheel probably is optimum for a 255 TIRE. A a 285 or 295 tire is optimum for that WHEEL, and that car.

Now if you have a 130hp, 2500lb car more tire is not the best solution for everything. Though I suspect more because the bigger wheel and tire is heavier that because it has the wrong stretch. And the whole stretch theory completely ignores the wide variation in sidewall stiffness across different tire compounds. I know back when I was heavily into autocrossing, all the stock guys ran 275 R compound tires on dinky 8" wide wheels (a good two sizes beyond the typical tire size recommendation) in my class and won National Championships that way.

Assume you have 10" wheels and a 3900lb car (with driver) with 400+hp. A set of Hoosier R7 in 255 will be faster than any other non R compound tire, 245, 255, 285, 295, 305 doesn't matter.

For a given car, the tire size is the primary limiting selection. So choose it next. For our big powerful car. The biggest you can fit will be fastest (no Miata too big tire issue here).

Now choose the widest, lightest wheel you can afford that hopefully looks good.

Thus a 285 tire on a 10" wheel or 305 on a 11" wheel are prime choices for the V8 Mustang, rather than a narrowish stretched 255 on 10" or 285 on an 11" wheel that the suggested rule of thumb would require. Now if we had more wheel and tire selection and a bit more clearance in certain spots maybe we would all be trying to run another half inch or inch wider wheel. But selection in 10.5 & 11.5 wheels stinks. And 12" isn't great either for the Mustang.

But feel free to run those "optimum" little stretched 255's. Just watch your mirrors so you don't hold me up with my sub optimal 285's.

I agree with you on multiple points. I think one thing that may be causing confusion is the generalization that all tire sizes are equal.. they aren't. Often times a 255 for one tire may be drastically wider than a 255 in another tire. In my case, a 285/35/19 NT01 has a thread width of 10.7" which ultimately will fit better on a 19x11 in my opinion. The 305/30/19 NT01 has a thread width of 11.8... while it may fit the 19x11 just fine and the shape of the contact patch with the extra width of the rubber will tend to allow for higher cornering forces (a lot of other variables assumed here)... I personally would sacrifice that contact patch shape for one that allows for slightly less cornering force (10.7" thread width), but gives me better steering response and feel ultimately allowing me to drive the car harder at the limit. If there was a more clear tire sizing standard that was consistent across manufacturers, then tire selection would be a lot more intuitive. For now... the best method seems to be to compare thread width numbers as that is the part of the tire that is actually touching the ground and seems to be the consistent measurement to compare actual tire sizes.

For all that is worth... I was running 305/30/19 RS4s on my 19x11s before... so unfortunately, I don't think it is as simple as saying xxx tire size should fit xx wheel across the board.

I have been wanting to pick up this book ("The racing and high performance tire" by Paul Haney) to read more on the subject as @NightmareMoon had mentioned earlier... the act of choosing a proper tire, contact patch shape behavior, etc...is really not discussed much mainstream nor understood fully by most.

 

[WildHorse]

There are many variables to selecting the proper tire size for xx wheel. RWD / FWD / AWD, 2% track time vs 98% track time, front / mid / rear engine, IRS vs live axles, etc., etc., etc. So it's always a compromise unless you have a 100% dedicated track car. And with that compromise IMO proper tire pressures play a more critical role. The average joe won't feel a difference if the tire tread width is .5" too narrow/wide for a particular wheel. But hey.. if you like stretch, for your viewing pleasure:

 

[CTH621]

My 285 on 11 inch wheel. Only a little stretch, tbh.

 

[ModularKid21]

Through my searches of wheel and tire combos, most people suggest a 285/35 for a 10” wide wheel. And essentially every 285 I’ve seen fits square, some even with the slightest bulge. I saw a couple pics where a memeber here put a 295/35 mpss on a 19x10 and there’s some sidewall stretch going on. It’s hard to tell he because like stated previously, not every tire is created equal

Edit; found it

https://www.mustang6g.com/forums/threads/mrr-m350s-with-295-35-psss.104930/

 

[WildHorse]

not every tire is created equal

True. Some nitto brands say a 305/30 or 35 is good to go on a 19x10 wheel.

 

[dmann]

True. Some nitto brands say a 305/30 or 35 is good to go on a 19x10 wheel.

Do you remember what types?

 

[wmfateam]

From my personal experience, ran a 255 RC-1 on a 17x9 on my 96 Cobra with stock weight/power and the next session ran a 275 on a 17x9 as well. 255 has slight stretch and the 275 had slight bulge. I went 1.3 seconds faster on a 1.06 second track with the 255 vs the 275. Same day, 1 hour apart, both brand new sets of tires. 255 had more direct steering and better corner entry/mid corner feel. 275 I could feel the "roll over" of the tire but had better corner exit. On my S550, I ran a 275/35 Federal RSRR on a 19x10 and then switched to an NT01 285/35 on those same 19x10. The NT01 having far superior grip the the little Chinese tire that could, was 1 second faster on a 50 second track. That was same weekend, Federal on Sat and Nitto on Sunday. Weight, suspension, power, tire brand and tire compound are all going to have an effect on what is the most optimal set up.

 

[WildHorse]

Do you remember what types?

Almost all there summer tire lineup.

 

[Norm Peterson]

So, here's a thought. For the 19x10 rim, why not go with a 265/40 AS3? Yes the tread is 20mm less than the 285, but it will have a very slight stretch that may really help compensate for cornering performance, yet provide a better daily ride. The 265 Michelin AS3 has 10.7" measured width and a 9.4" tread width. This puts it very close to the 1/2" more narrow desirability vs running a wide 285.

Does that make sense?

"Iffy" at best. One reason being that when you stretch your tires out toward max recommended you pay for the increase in cornering stiffness with a decrease in vertical compliance (IOW, stiffer).


Norm

 

[Norm Peterson]

As I talked with a guy who tracks his Cayman and BMW quite a bit, I started understanding some of the theoretical mechanics behind why a "slightly" stretched tire may actually contribute to better handling, and a more comfortable ride for a lot of European cars. It makes sense, but only to a certain point, and the amount of stretch can quickly become very problematic vs a benefit as it stretches further. This idea may also be why the GT350 had a slightly stretched tire from the factory

Steering response and precision benefit as wheel width goes up and the tire size (possibly tread width) is held constant. That's a plus, because getting the turn started is kind of important. To a point, that's perhaps more important at the HPDE level than maximizing ultimate grip.

I certainly question the matter of ride comfort improving as tires are stretched, or stretched further. Ride quality doesn't have to get appreciably worse, but it's not going to improve. The straighter you make the sidewall shapes by stretching the bead seats further apart, the less flexible they become. And sidewall flexibility in the vertical direction is your car's first line of defense against the 'sting' of hitting bumps and other roadway roughness.


Norm

Sponsored