Price: $400

Mavic's Speed City wheels, disc-brake-compatible 700c wheels with the 135mm rear-axle spacing common to mountain bikes, allow riders to put easy-rolling skinny tires on their disc-only-equipped mountain bikes in a flash, far quicker and easier than removing knobbies and putting slicks on your 26-inch mountain bike's rims. The Speed City 's larger diameter rim, combined with a low-profile road tire, should drop into most disc-equipped mountain bikes, even full suspension, with no clearance problems.

But a few caveats. One: Even if you have the proper-size disc rotors, there's still a chance that the rotor and cassette positions on the Speed City wheels won't match those of your 26-inch wheels. These dimensions are supposed to be standardized, but they often differ from one hub manufacturer to another. Two: The wheels do not work on mountain bikes with rim brakes.

Though somewhat heavier than most high-end 26-inch XC wheels, a possible weight gain in the wheels is offset by a dramatic weight loss in the tires. Combined with the super-low rolling resistance of a skinny road tire, your mountain bike will feel turbocharged with these wheels. The wheels are reasonably stiff and can take some pothole abuse without needing attention. The hub mechanisms are similar to those used by Mavic in their Ksyrium road wheels and CrossMax mountain wheels, which have traditionally held up well for us.

If you have only that disc-equipped mountain bike, and you jump back and forth from dirt to pavement several times a week, the Speed City is probably a worthwhile investment. Otherwise, it's a big price to pay for some occasional convenience.

Best Thing: Turns a dirt machine into a street rod

Worst Thing: Big price to pay for convenience

Our Verdict: If you need them, you'll know it

Price: $400

Weight: 881.5 g (front); 1,128.8 g (rear)