Speed is a stat related to the top speed of Karts, ATVs, or Bikes. The higher the stat is will determine the maximum speed a given kart, ATV or bike can go. Usually heavyweights tend to have the highest stat, while medium-weights have an average stat, and lightweights have a low stat. Generally, speed is reduced during turning, but drifting negates some of the speed loss. Some Vehicles that have a very high Off-Road have low speed. So Off-Road is the opposite of speed.
Mario Kart Racing Wiki