MPH
1-9 = Auxiliary Thrusters Only
10+ = 1/4 Impulse
20+ = 1/2 Impulse
30+ = 3/4 Impulse
40+ = Full Impulse
55+ = Warp 1
60+ = Warp 2
65+ = Warp 4
70+ = Warp 6
75+ = Warp 8
80+ = Warp 9
85+ = Warp 9.5
90+ = All Good Things must come to an end...
As good a speed scale as any for warp-capable passenger vehicles, but.... I see a problem when you are just running errands in town or spend long periods of time in rush hour traffic. Traveling at sub-light speeds for extended periods of time force even futuristic starships to occasionally reset their ship-board chronometers to account for relativistic time dilation. It's possible that if you get jammed in the evening crush, you could miss the game/Lost/American Idol/insert favorite primetime programming here.
Be careful. While we can have fun bestowing warp drives on our minivans, I'm convinced modern vehicles would never withstand the rigors of time travel in a futile attempt to travel back to see what you missed while grinding away at half impulse.