Tire Speed ratings
 

Speed rating	Miles/hour	Kilometers/hour	Application
N	87	140	Spare Tires
P	93	150	-
Q	99	160	Winter Tires
R	106	170	LT tires
S	112	180	-
T	118	180	-
U	124	200	-
H	130	210	Sport Sedans
V	149	240	Sports cars
Z	149	240	Sport Cars
W	168	270	Exotic Sport cars
Y	186	300	Exotic Sport cars

thanks to 4jacks for the follwing information:

Tire Size ###/##/##
The first Three Digit Number is the Thickness of the Tire in mm
The second Two digits is the aspect ratio of the width and the height of the sidewall
The third Number is your Rim Size.

Also I found the following link really super nifty when buying my rims. I'd hate to link to it as a sticky, maybe their is someway to steal it I dunno, check it out i'm sure you'll think it's nifty as well

It calculates how your Odometer and Speedomer will be affected, and everything.

http://www.2k1civic.com/stuff/tire_calc.cfm

Heres a working tire calculator.

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

Thanks to JudoAnt for the link.