A wonderful game that has a lot of potential! The 4th stage did have terrible performance, while the 3rd stage could use some visual touch-ups just to bring it up to par with the others in both clarity of route, theme and overall polish. The two most important features I'd like to see implemented, however, are a settings menu to mute the music (no matter how good it might be now, it'll start to grate after a while) and a way to tell what gear you've just entered without looking down. An audio cue could work, e.g. pitching the gear switch sound up/down.
If you do decide to work on some more content for this, I think a chase sequence in a city is kind of a must. Weaving between traffic, then building speed to narrowly avoid a collision with a train, all while pursuing/being pursued for whatever reason. The pieces are all there and this Chekhov's gun begs to be fired.