RyanPerrella

16" wheel vs 18" wheel? The 18 is 2 inches taller and that makes tire circumference much larger. Again aspect ratio is proportional to tire width. Aspect ratio has no relation to wheel size. If your going with an 18" wheel, or plus 2 on an S4 your aspect ratio needs to go down to have a similiar circumference to the original size, or yes you will have fitment problems, and speedo's will be out of whack (rear wheels)

Based on my above hypothesis of 1 inch in diameter is equal to 5% in the aspect ratio

If stock 17" sizes are 225 45 17, then going to a 50 aspect ratio on a 16" tire should be the same overall circumference. (225 50 16 is the factory tire on the GT) Similiar to a 40 aspect ration on an 18" wheel.

a 225 45 16 tire would be too small, look at the 1" to 5 ratio that i have suggested. It works when tire widths are equal or close +/- 10mm.

dprantl

Agreed. Now wouldn't it be nice if the calculator could tell me if a 235/40-18 or 245/40-18 tire would fit on an ET52 wheel? Or an ET56 wheel? Or I could mess with the offsets to see at what ET it would not fit. I think that would be a great addition to an already useful tool.

Dan
'91 928GT S/C

Brett928S2

Hi Ryan

I had a very particular reason for wanting a 245/45/18...it was to match the rolling radius of my rears....which are 275/40/18...

Those 2 sizes were an almost perfect match with rolling radius...

Fronts = 83.82
Rears = 83.75

The reason was for my 200 mph plus runs....that rolling radius OVERDRIVES my gearing by 7% by the way (useful on top speed runs)...lol

For normal road use I wouldn't have gone that large on front or rear...

All the best Brett

RyanPerrella

well my point was that when you know what does fit, you can then pretty much assume what doesn't couldn't you?

tire size has no real issue when it comes to offsets. If the offset is not correct te wheel wont fit or can be made to fit but at the expense of the way it looks.

Why are you so interested in knowing every possible tire size that fits? When you know the size you buy the tire? Anything can be made to fit as i stated, but you want the CORRECT tire, and the correct tire is one of a few sizes, you know what they are, you buy that size tire, why would you want to fool around with different aspect ratios and widths and offsets if you already know what fits, why mess with it?

I guess i just don't understand your logic for wanting to change aspect ratios and basically see how different a tire you can come up with that will fit and not rub? I guess i am asking you WHY?


Gotcha Brett, good luck with 200 mph (not sure if you will find a tire thats going to be safe for that speed with that tall of sidewall though)

I know Michelin has special tires for high speed running and to my knowledge they don't come in a 45 series aspect ratio.

dprantl

Well, for example I may want to run a larger diameter in the rear (larger aspect) to offset the short gearing on the GT gearbox. Or, in the case of 235/40-18 which is a larger diameter, and 235/35-18 is a very rare (read $$$) size. Or, a 235/35-19 may be too little cushioning for comfort so a 235/40-19 may be better for some. There are many examples.

Dan
'91 928GT S/C

JPTL

Brett,
With all due respect, offset has no effect on whether dust caps will protrude too much to prevent fitment of center caps. It's the thickness of the wheel-to-hub interface at the bore that dictates that.
(how's that for a stick in the bee-hive?)

Vilhuer

No point at even trying to make any calculator to take into account cases where circumference is changed from original. If someone is willing to change it what are the limits? Make them too small and thread will not touch the ground. Make them too large and they can't be mounted in at all.

928 like all cars are designed to use some specific diameter tires. Whole reason for plus sizing concept is to keep diameter same while width and wheel diameter changes. In 928's case design parameter was 225/50-16 and every one should try to keeps diameter close to it. Even when front and rear tires are different width their diameter must be kept close to same for ABS brakes to work properly.



http://www.tirerack.com/tires/tirete....jsp?techid=25

Brett928S2

Hi

Lolll...don't complicate things even more, although I did not know that

I "assumed" it was the offset....

All the best Brett

mark kibort

Thats not entirely true. in fact, the spedo's are off anyway, and with 305 35s, its dead on (26"). (in the rear) the fronts i run are near stock diameter 24.7" with the 275s, and the disparate diameters are no issues for the ABS system.

Mk