I have read a lot of threads and listened to a lot of talk. But I am still a little unclear about tire/wheel sizes and how they effect our Gs.
If I want to run a 245 up front and a 275 in back, what profile will I have to run to keep close to the 18" stock diameter . . . with a 19" wheel? With a 20" wheel?
I don't want to run into VDC troubles or big speedo inaccuracy trouble. Which combo, with what profiles would be best?
I would like to do a 8.5x20, 10x20 setup, but if that is asking for trouble, I'm flexible.
Can those who are wheel savvy, chime in and help me out?
Thanks

 

Why not just find out for yourself?

http://gs.tolan-hoechst.com/tirecalc.htm

Remember you want to stay as close to stock tire height as possible.

 

Thanks for the link. But wow, that's a lot of numbers.
I'm no math major, but from what I could make out. If I want to run 20s, 245 and 275 combo, I am going to be looking at 30 profile tires? If I want to run 19s, I can go a 35 profile?
Is there a big difference in ride comfort between a 35 and a 30?
Thanks

 

I'm a little confused from all that you said, but here's what I take:

you want 20x8.5 in front
you want 20x10 in rear

I'd suggest 245/35s in front which will give you 1.24% different
and 285/35s in rear which will give you 0.52% diff (speedo will be off 0.5% which is within the noise of the tolerance on it's accuracy anyways I would say)

If you're stuck on going to 275s in rear, you can still use 275/35 and it will give you the same 0.52% off, but it will be off -0.52%. However, for tire/wheel width ratio consistency, I'd go w/ the 285s.

I can't say whether these will all fit within the wheelwells or not though. You haven't mentioned offset or wheel type.

 

Here is another, simpler tire calculator.
http://www.miata.net/garage/tirecalc.html
Ideally, I've heard you want to be within 3% of the original tire rolling diameter.

 

Thanks for the alternate calculator. The visual is nice. The 3% number is what I was looking for. I don't want problems with the VDC or anything else. I would like to run 20s, but I am a little afraid. 19s might be the more prudent choice.

"mkerley" - When I run the numbers with 20 inch wheels, 30 series tires appear to get me closer to stock than 35 series. 35 series looks like it works better with 19 inch wheels. Am I reading this wrong?
Thanks again.

 

You're calculations for the 20s are correct.

 

Doh... my bad. forgot to change the wheel size from 19 to 20 on my spreadsheet.

for 20s...

wheel size...........tire size................%error
8.5x20..................245/30..................-1.15
10x20....................285/30.................0.06
10x20....................275/30................-0.83 (wheel/tire width ratio a little high)

So, yes, you're correct. 30s appear to be what you'd need.