F451

Hey gang,

I've got two questions.

1. Do any of you know how much the diameter of my tires will change if I go from:

225/50 x 16" (7 x 16 wheel) to 245/45 x 18" fronts (on 8.5 x 18 wheels).
245/45 x 16" (8 x 16 wheel) to 275/45 x 18" rears (on 10 x 18 wheels).

This tire calculater is giving me some results that do not seem correct:

http://www.miata.net/garage/tirecalc.html

2. Do I take into account the diameter of the tires when I do my ride height adjustment? I'm thinking no, but would like to know what people are doing with their larger diameter wheels sets like 17, 18 etc.

Thanks!

Ed

LT Texan

the fronts increase about 1 3/4" in height and the rears about 3"

The ride height is set independent of the size of tire.

ShawnSmith

275/45 x 18" is an awfully large tire for the rear.

I think you'd want either 275/35-18 or perhaps even 295/30-18 to get closer to the right rolling radius.

Jim bailey - 928 International

The goal is to keep the tire diameter the same by reducing the sidewall profile. That way the speedometer is accurate and ride height stays about the same.

Mike B

I went with 225/40/ZR18 for the front and 285/30/ZR18 for the rear...which is pretty darn close the the original diameter / circumference, etc....hope that helps.

F451

Good info guys. Thanks.

Tom928

Not meaning to hijack - just understand.
If I increase the wheel OD, and the tire ID (or sidewall) decreases to keep the same overall diameter. Doesn't the ride height stay the same?

F451

Yes, that is my understanding.

What has me stumped though is the difference in the particular combination that I'm looking at. I'd really love to measure the diameter of the stock 16" wheel with the stock tire on it, and then measure the diameter of the tires I'm looking at.

If in fact the tires that I'm looking at have a significantly (3" on the rear according to one poster) larger diameter then the stock tires, and in that case, my speedo would be off (no biggie).

But more importantly, I'm wondering what to do about the ride height. Some are saying I should adjust to the stock ride height specs, and others are saying to adjust the ride height so that the "stock height" suspension geometry is the same.

And then I have Mike B who mentions his tire sizes are very close to stock, and his tire sizes are very close to the ones I'm looking at.

I need to measure these things.

And I can't believe I gave away my extra 16" wheel set when I sold the '85. I never touched them in 1.5 yrs, and now I could have used them twice in the last few weeks! Grrr!!!!

F451

Ok, who can run outside and measure the diameter of their 16" wheels with the stock sized tires on them?

Mike B

Ed, these are the wheels I picked up. They arrived in no time and in perfect condition. I would buy wheels from Carlos again.

http://cgi.ebay.com/ebaymotors/Porsc...4350QQtcZphoto

Garth S

Most all Porsche wheel sets 16" to 18" inclusive are fitted with tires of an aspect ratio ( sidewall height) to achieve a max. overall diameter of 25".

Stay with 25" od, and the suspension geometry will be happy. There's a zillion posts on the other two issues - section width and offset (ET).

largecar379

an easy way to check tire diameter differences is just go to Tire Rack's website and pick any tire brand, then pick the size you currently have, then pick the size you want to upgrade to, and compare.

they have the diameters listed on one page so it's easy to research......

As many have said, tire sizing does not change vehicle ride height adjustment. Bigger diameter will, however, change the height of the car from the ground.

Sterling is correct about not changing ride height adjustment if possible.

and yes, as I have said before, you can go as big as:

18 x 10 front (may require fender work)

19 X 11 rear (no fender work required, but you can roll the lips slightly just to be safe)

IF you are trying to use a larger wheel than 19 X 8 on the front (using a factory Porsche wheel), you can't because of offset's.....you must go to an aftermarket wheel---as far as I know.

NOTE: there are specific offsets to make these work for OB and later 928

Carlos @ Wheel Dynamics can guide you through your specific needs (what will fit and what won't), and he's more than willing to help.

See my avatar----

----Russ

F451

Great info. Thanks guys!

Dwayne

Ed,
I have a set of stock flat disk wheels and I just put on a set of new stock size tires TODAY (225/50ZR16). I measured for you. With weight off wheels, the diameter is approximately 24.75" - very close to what Garth posted. If you take the theoretical measurement, you get 24.86" (16" wheel + tire sidewall height (225mm * 0.039 - convert to inches = 8.86")...16 + 8.86 = 24.86". Approximately 25".

F451

[edited to get the math correct - after Dennis helped me out]

Excellent, thanks Dwayne! I was just about to "do the math" using that same basic formula from Tire Rack:

Width x Aspect Ratio = Section Height
Section Height x 2 = Combined Section Height
Combined Section Height + Wheel Diameter = Tire Diameter

So, lets see if I can figure it out too. For the '88 S4:

Stock Fronts: 225/50 16

225 x .50 = 112.5mm = 4.43"
4.43 x 2 = 8.86
8.86 + 16 = 24.86"

Stock Rear: 245/45 16
245 x .45 = 110.25mm = 4.34"
4.34 x 2 = 8.68"
8.68 + 16 = 24.68"

Ok, so like Dennis says, lets call it 25" for the stock S4 wheel/tire setup.