Miata Mailing List: August 1999, Message #334

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The three numbers describing tire size are:
   T - Tread width in millimeters
   S - Sidewall height as a percentage of tread width
   W - Wheel diameter in inches

Yes, one dimension is metric, the other is archaic. Go Figure.

So the stock Michelin Pilot listed as 195/50-15 is 195mm wide,
  has a sidewall height of 97.2mm, and fits on a 15" rim.

To figure the diameter of a tire:

  T * S/100			Gives you the height of one sidewall in mm.
  2 * T * S/100 		Gives you both sidewalls in mm.
  2 * T * S/100 / 25.4		Gives you both sidewalls in inches.
  (2 * T * S/100 / 25.4) + W	Gives you the whole diameter in inches.

  (T * S / 1270) + W		Is the same thing with the constants combined.

So the Hoosiers are:
  (225 * 50 / 1270) + 14 => 22.86"
  (225 * 45 / 1270) + 15 => 22.97"

And the Pilots are:
  (195 * 50 / 1270) + 15 => 22.68"

The Hoosiers are about 1% bigger than the Pilots. You'll have a teensy
  bit higher gearing, and your speedometer will read a teensy bit lower
  than it probably already does. But you probably won't notice either.
  You *will* notice the improved grip. :)

  Keith Hearn
  '99 10AE "Sexy Sadie" the Sapphire Shark
  Milpitas, CA

In message <37A71BDD.2D2399FF@mediaone.net>, william schenker writes:
> Any idea what the diameter of the stock Michelin Pilots is?  I ask
> because I'm thinking of going to either a 225/50/14 or 225/45/15 Hoosier
> for autoxing - both size Hoosier have a stated diameter of 22.7".
> 
> I'm trying to figure out if I'll have enough clearance of the wheel well
> compared to what I have now (the Pilots.)
> 
> TIA
> 
> Bill Schenker
> 
> 
>