So I've gone through the speed classes for Van-Uber (I know, DLC team, but I like the look of it.)
With a little bit of approximation and rounding up:

Vector is a slow crawl at 370km/h
Venom averages a nice leisurely 450km/h
Flash jumps that up to around 530km/h
Rapier is something like 650km/h
Phantom is around 790km/h

Obviously it's hard to pin it down due to the speed loss while turning and airbraking, and the sudden gain while using boost pads, barrel rolls and turbo's.
Also, obviously, teams with Speed 5 will average higher speeds, while teams with Speed 2 average slightly lower speeds.
And while the difference is never much more than 10-15 km/h between a S2 and S5 ship, it makes quite a difference on straights where the faster teams will slowly squeeze past the slower ones.