mourat

I guess this is more of a question for one of the sponsors, more of a technical question I can't just find googling.

Does anyone know how the overall tire diameter is affected when putting a narrow tire over a wider rim?

Example. Tire diameter of a 255/35/19 is around 26" inch.
One is to assume that is only true if the tire side wall is square with the wheel, ie. Installed on a 8.5" wheel.

If I were to put a 255/35/19 over a 19X10 wheel, that according to most sites is the smallest recommended tire width for a 10" wide rim.

So one would think since the tire's side walls will stretch outwards, this would decrease the overall diameter of the tire since its mounting surface was stretched outwards .75" on each side.

How much does that affect the overall diameter, is it safe to say I would see atleast a .25" overall diameter decrease?

Thanks!

mourat

Ok so found a thread on some VW forum, stated that the rolling diameter does not change when stretching a tire over a wide wheel, reason being because the tire is made of re-enforced steel belts, which cannot compress, the cross sectional width of the tread however does, which makes sense.

Hope wheel / tire expert can confirm. Techincally my 255/35/19 tire being 26.02" or so, will be 26.02" whether its on a 8.5 or 10" rim! Please confirm, if so I'm a happy man.

haljensen

Not an expert but;

Since it's a calculated diameter where wheel width isn't in the calculation then the calculated diameter would remain the same.

Since the calculation depends on the aspect ratio and the aspect ratio % WILL change with wheel width then actual tire diameter would change with wheel width.

The only way to prove it would be to mount the tire on a 8.5" wheel, measure the diameter, then mount the tire on a 10.0" wheel and measure the diameter again and compare the difference.

mourat

After thinking it about, the tire shouldn't be able to shrink, the steel belts inside will keep it same, the side walls would just come up slightly as they stretch, the tire itself would remain. otherwise you would need to compress the treat as well which is not going to happen.

Le Mans Z06

In theory though couldn't the sidewall be compressed down a tad? There are no belts in in the sidewall therefor I still see it being possible.

glass slipper

Section width does change with rim width but since tire diameter doesn't change, the aspect ratio is what changes. The tire section width and aspect ratio is valid only for the specified rim width.

mourat

Found best info right here:

http://www.yokohamatire.com/tires_10...io_calculator/

Because the overall diameter of a radial is determined by its belts, this tire's overall diameter should remain virtually unchanged.

Aspect ratio changes, on that site tells you how to calculate.

Audacious Nick

Why would you mount a 255 or 265 on a 10.5" tire in the first place is a better question.

mourat

255's on a 10" wheel is what I'm looking to do. Reading up on what brand tire to use, some stretch better than others. I have 19X10 in the front ZR1's, I want to run 255/35/19. First, its 26.02" so it will raise the car a bit. I can't run 285's cuz will touch fenders I dont have widebody up front. and running 265/30/19s the diameter is only 25.25 so its low to the ground. Going to 255/35/19 will go up to 26.02 raising the car and filling in the wheel gap.

Also 255's should steer much nicer and not follow the grooves on our Vancouver broken roads. Also 255's so long as they stretch nicely and I don't think i'm asking for a lot putting 255's on a 10" rim, it will let me lower the car slightly and not worry about fender clearance.

haljensen

Raising the car and filling in the wheel gap?

26.02" Diameter
-25.71" Diameter
= 0.31" Diameter difference

That means raising the car 0.155" and filling the wheel gap by the same amount. Hardly measureable, let alone noticeable.

mourat

No right now I have 265/30/19s in the front, thats only 25.25", I bought my wheels with these mounted on them so 0.385 will be raised by. Thats noticeable. Plus i'm putting a small spacer on the wheels to fit z06 front brakes on so I need to tuck the tires in some.

timd38

Why not just get the correct size wheels? They may fit, but they will not look right.