Kett-Cars are awesome! We have owned 5 and still have 4. The frames vary in size (I have seen small, med, and large). The smaller ones have rubber treaded plastic tires and the large ones like this Woot have pneumatic ones. My son has outgrown the smallest one which was gifted away, but he still enjoys the mid size ones.
I have 2 of the larger pneumatic ones, older versions of this Woot, and at 5'9", 155 lbs, I can fit on them and race my son, albeit lose.
The gear shifter allows freewheeling downhill and the brakes work well. Handling is superb - corners like it's on rails with responsive hook-and-groove steering.
These last forever and are easy to tinker with and tune-up if you're somewhat mechanical or have experience with bikes. Other than rust there's not much that can go wrong, and I do advise storing them indoors when not in use.
Best of all they're kid-powered and great exercise! They are expensive but German engineered and high quality. However I don't need another.