How are tire sizes calculated to maintain the overall diameter with larger wheels?


ViperJoe

ViperJoe

GT Owner
Mark II Lifetime
Aug 17, 2006
1,305
Washington Crossing, PA
I'm considering replacing my forged BBS wheels for HRE 545's. IMO, just too much sidewall in the OEM configuration ..... and ..... I would have tire options which we presently do not. I'll either go up 1" or 2' in wheel diameter, maintain the same width (or perhaps go a touch wider) and of course maintain the same offset. All input appreciated.
 
Well, it's not too hard tofigure out overall diameter. I usually will pop open Excel and enter the following into an equation. Let's use our existing rear as an example.

315/40-19

The tread width is 315mm.
The sidewall height is 40% (of tread width) = 315mm x 40%
The rim diameter is 19 INCHES

So, to compute overall diameter, you have to add two sidewall height (top & bottom), convert the result to inches so you can add it to the rim diameter.

Diameter = ( (315 x 40%) x 2) / 25.4 ) + 19
 
Easier yet - ask the tire guy. They will have height and tread width (and maybe side wall) as well as thread depth, traction, rating, and heat dissipation as standard information for any tire.

A no brainer for the mathematically challenged :wink
 
nota4re said:
Well, it's not too hard to figure out .....
Click to expand...
That was easy, thanks!
 
Tire/Wheel Change Corrections/Specs

ACRJoe said:
I'm considering replacing my forged BBS wheels for HRE 545's. IMO, just too much sidewall in the OEM configuration ..... and ..... I would have tire options which we presently do not. I'll either go up 1" or 2' in wheel diameter, maintain the same width (or perhaps go a touch wider) and of course maintain the same offset. All input appreciated.
Click to expand...

Just go to the Tire Rack site. They provide technical data for nearly all available manufacturers' tires, including revs/mile which will give you the speedometer correction for the computer.

www.tirerack.com
 
 
Here is a site that calculates it for you

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