Terry - one thing that I am seeing is that the 265/40 tires of any interest here tend to have actual tread measurements that are essentially the same as 285-wide tires and frequently wider than the 275's. Is the structure of the 275's and 285's enough different to matter, is it the ability to work with wider wheels, or is it something else that I'm missing completely?
If it matters, I can't get very excited about mounting 285's on wheels that are only 10" wide. Particularly not after running the 18" GT500 front tire setup (255/45 on 18x9.5) all around.
Norm
The section widths might be high on some 265mm tires, but the tread width
should always be higher on a 285mm tire. Of course some tire companies "fudge" the numbers a bit, but generally they do that on all of their tires, and I've rarely seen them off by more than 10mm (there are a few notable "outliers" with extreme presidencies).
I am a bit confused about what you are saying.... do you think a 285mm tire is too
narrow for a 10" wide wheel? I will assume that's what you meant, since the 18x9.5" wheel you talked about was running only a 255mm tire (that's a stretched fitment).
285/30/18 on 18x10" wheel on S197 Mustang
I've run 285/30/18 tires on a lot of 18x10" wheels on a lot of cars for a lot of years. This used to be the "go to" tire/wheel combo for all of my autocross and track cars... and a lot of other people's, too. Super common tire size in performance tire models. Unfortunately this tire is uber-short (sub-25" tall) for the S197 Mustang, and we've seen big performance bumps by going wider and wider and wider with these heavy cars.
The 18x10" wheel + 285/30/18 tire combo is very common in the BMW racing world
There are ranges of wheel widths that work with various tread/section widths. I tend to go wider on the wheel than most charts say is "ideal, but I'm still very comfortable with a 285mm tire on "only" a 10" wide wheel.
This 285/30/18 tire was a S-T-R-E-T-C-H on these 18x11" wheels, however
Generally, 285mm tires work well on a 10-11" wheel, and I suspect a 10.5" is ideal. This size is a bit of a squeeze a 9.5" or 9" wide wheel and look downright silly on an 8.5", but I've seen it done (wouldn't do it). Problem is - there ain't an 18x10.5" wheel made that fits the S197 chassis, even remotely well (the 18x10.5" Enkei some folks use on these cars
stick out of the rear fenders by a mile). But the 18x11" wheel is a bit too wide for this tire width, as we've seen first hand (E30 above). That's a 11" wheel width more suited to a 295 or 305mm tire. For a 315mm tire I like to go 11-12" wide wheel, and I'm switching to all 12" wide wheels at all 4 corners on my own S197 (we run a 315 on an 11" front, 12" rear now).
The real key issue here is:
wheel availability. The biggest "Easy" fitment wheel for the S197, one that fits both the front and back for easy tire rotation (same offset/no spacers/fits under the fenders) is the 18x10" wheel. We sell the D-Force and Forgestars in this size. Anything wider has compromises in some way (either sticks out past the fenders or has wildly differing offsets front to rear, or needs lots of camber to fit, or all of the above). Once you've settled on the fact that this is the widest wheel you can practically run "square" on an S197, your tire choices dwindle to this list:
- 275/40/18
- 285/35/18
- 295/35/18
Those are the 3 tire widths that fit this wheel, and the most common sidewall ratios in those widths. I've run with all 3 of those tires on the 18x10" wheel on these cars, and each one has its pros and cons. The 295mm tire sometimes needs a 5mm spacer up front. The 285/35/18 only comes in a few brands. The 275/40/18 is the tallest of these 3 (and closest to stock height for speedo calibration) but is also the narrowest.
And like I've said 1000 times - these cars are freagin BIG and HEAVY, so run as much tire width as you can afford to. Don't piddle diddle on 8.5" or 9" wheels, go to 10's or don't bother. The only reason to use narrower wheels on these cars in motorsports is for rules limits (STX limits to 265mm tire/9" wide wheel, American Iron limits to 9.5" wide wheel/275mm tire, etc)
People ask us all the time, how we manage to put 2-7 seconds/lap on the American Iron track records, when we are running the same power-to-weight ratio? The answer is ADDED WHEEL AND TIRE WIDTH. This wins on big, heavy cars.
That's my two cents...
A better bet is a wider wheel and tire package, even with street tires...
