Not normally true
Normally ever kind of race car, be it offroad, or on road, is usually faster than any race bike. I am a true bike lover, but I feel the need to dispell a lot of the rumors about how quick cars and bikes are, or aren't, on the track. Take WSBK, or GP bikes, Formula one cars will lap those kinds of bikes on the 3rd or 4th lap on most tracks. As for offroad, it has to do with the terrain and a lot of chance. Almost always a car will win offroad, but there are times, generally twisty and lots of odd terrain, favors a bike.
There is a lot of common misconceptions about bikes being the fastest things around, we just do it so elegantly, and smoothly, but you can't be the contact patch of 4 wheels, you can't beat the ability to slide in the turns, you can't beat the stability of 4 wheels over 2, and you can't beat the braking power of 4 big tires on the pavement. Nevermind the fact that bikes can't even hit their potential on the brakes because of the rear end coming up.
Time to put on my suit, here come the flames!
Rob