It seems there is potentially a little technique you can do with manual transmission. If you shift up at (or near?) the highest RPM for a given gear, it seems to automatically boost you up near the top speed for the next gear without having to accelerate through. This game is not exactly responsive to control inputs, so doing this consistently has been difficult, but I think there's something there. I think you can use this in several places to regain top speed without having to accelerate back to full, but it will likely require precise timing.

Watch the tachometer and speedometer at ~19 seconds for an example of what I mean, though you can get a more dramatic "jump" in speed if executed better: