>     What are "HR" rated tires?

HR is the guaranteed speed rating from the manufacturer. If you treat the
tire the way it asks on the sidewall (don't exceed the maximum
load or pressure), then the tire is gauranteed not to overheat for all
speeds up to a certain limit.

For HR rated tires, it's 130 mph. For SR, I believe it's 113 mph.
For unrated tires (165R15 etc.), I believe it's like 95 mph.
For VR, it's around 150 mph. And for ZR, like what people put on
Corvettes, it means there is no limit listed. Tho I'd imagine that
breaking the sound barrier would probably make them unhappy. :^)
Tire dealers can give you the exact numbers.

This higher speed rating, comes mostly from making the tires
less flexible, usually by stiffening the sidewalls as David Schwarze
mentioned. When a given patch of tread comes around and contacts
the road with the weight of the car on top of it, it flexes to meet
the road, and usually the sidewalls above it flex too. This flexing
generates heat inside the tires, which has no easy way to escape,
and so tends to build up. Steel Belts tend to make this problem
worse.

Sidewalls that flex less, lessen this problem, and so such tires can
have a higher speed rating. They have the added advantage that they
corner better, and control the weight of the car better as the car
swings and sways around on the freeway.

The bad news is, stiffer sidewalls also tend to make your ride
harsher. Tire design is a compromise, where you often trade better
handling for stiffer ride, or trade a softer, better-gripping tread
compound for increased tire wear. You pays your money and you takes
your choice.

 -Steve Maher     smaher@gi.com     '71 VW Transporter
                                   '66 Mustang Coupevertible
                                   with 215/60HR14 tires that
                                   _noticably_ improved handling
                                   over the POS 195/70R14's it had