search by size and have the old sizes available.
user modifiable sort patterns (size, brand, price)
if it's search by bike, be damn sure to have your database in line (tblBike, tblTire, tblBikeTireXref)
can't remember if you have it, but a "remember my bike" option
the one thing i _hate_ is when things are only listed by brand. you guys aren't that way as there's typically the 3rd option (show me everything in the category) but a _lot_ of sites make you click avon, realize there's nothing for you there, click back, click bridgestone, realize there's nothing there for you, etc....
all in all, dennis kirk isn't too bad, though your search isn't the best....maybe integrate google for sites.
might think of more later.
also a web programmer.
more backend than front.
[edit]
oh yea, if you could make it so the shop doesn't charge you so much when you bring your internet tires in for mounting and balancing, that'd be cool too.