To improve the various vehicle skills, simply by driving or riding a bike or such, just do it instead of doing missions. Just drive around the map. You'll get it to 100% skill level. Though by doing the driving, boat, bike, and flight school, you can get it to 100% by getting all gold in those. You'll still have to ride around on a bicycle to get 100% on the skill level. To go top speed on a bike, if you're using a console, then most likely you just need to go slightly forward on the left analog stick to lean forward. If you lean forward too much, the effect will be negated. If you're using PC, then it's just whatever key is set for steer forward/down. As for driving to top speed in a car, just hold on to w or the throttle button, which by default is w.