Why are some tires (for my KLX250S) not approved for highway use? Probably a stupid question.

 

The manufacturer chose to not run them through the $$$ approval process. Some years ago I wanted a knobbier rear tire for my KLR250. Lots of 4.60 x 17 available in Europe but not imported to this side of the pond. Found a Kenda (don't recall which one) in the right size but non-DOT. that same tire in other sizes was DOT. Go figure. Ran it anyway. Put the DOT Kenda 270 when I sold it, included the non-DOT with the sale.

 

I've ran both and couldn't ever tell it made a difference, I've never had an officer check to see if I had DOT or not.

 

There are plenty of DOT options on tires so there's no reason not to run them.

If you're looking for aggressive knobby tires, check out Pirelli Scorpion XCMS front, and for the rear, look at Pirelli MT43 or a Dunlop D606 rear. They're DOT certified and last fairly well on the road.

If you're looking for something less knobby, there are countless options.