"Speed up" is the most general phrase I'd use:
You do know there's a line of a dozen cars behind us? You should probably speed up.
If you're looking for something a little more descriptive, then
step on the gas
give it some gas
However, if you want to tell someone to accelerate as fast as possible, then these work:
put the pedal to the metal
hit the afterburners
and various others.
(edit) It's much the same for motorcycles, except for "step on" the gas or put the "pedal to the metal", neither of which apply. Instead you might say something about giving it "full throttle" ... but, when you're riding passenger on a motorcycle, unless you have an intercom system it's hard to communicate. So I'm not sure you can say anything.