its on the Miata site that was linked to from the Maxima site where I was trying to sell my Maxima that my 1.8T replaced!
:-)
anyway... its kinda neat...<ul><li><a href="http://www.miata.net/garage/tirecalc.html">http://www.miata.net/garage/tirecalc.html</a</li></ul>

A larger tire size yields a larger outside circumference - this causes the tire to cover more ground with one rotation of the tire.

A mile is completed with fewer rotations of the tire. The speedo is showing you the speed based on a certain number of rotations per mile. When the speedo says you are going 100mph, you are actually going faster (105, 110, 130 +++)

Some more theories if I remember physics properly.

There is also an effect on your gearing ratios. The engine usually has to perform a certain amount of work to spin the wheels a certain rate (speed vs. wind resistance, etc.). Since the tires aren't turning as much to achieve certain speeds etc., the engine also isn't turning as fast (or you've stayed in lower gears longer). Since the engine is spinning slower than designed, it may have extra stress on it - or not produce the amount of torque required to get the job done. The engine will feel sluggish and underpowered as a result.

If the tire size is larger you would want gearing that allows the engine to still spin at it's optimum designed speed - a whole transmission replacement is not a practical possibility for us normal folks.

My .02,

Gordon Martin
98.5 2.8QMS (Santorin Blue)