[Varyon]

If you have two abilities (be they due to actual Advantages, spells, technology, or whatever) that each grant you some degree of Air Move, you'd be capped at the higher speed. If you have an ability that grants you Air Move, and something that normally wouldn't grant you such is giving you a "push" (or pull, as the case with vitruvian's case of a car pulling you along like a kite), you'd again use the higher speed, and if the push is faster than your Air Move, you'd need to make Control Rolls (penalized based on how much faster it is) to avoid, well, losing control and crashing to the ground. Note a human being pulled along by a car would do the same thing to try and keep up by running (as all you have to do is lift and lower your legs fast enough while keeping your balance, instead of also needing to provide the motive force, you can go faster than your actual maximum sprinting speed when pulled along by something faster, but you're likely to eventually trip and fall).

I don't think GURPS actually has rules for how to handle the above situation, but taking a page from "Pushing the Envelope" (B395) might work. Requiring a roll at DX+3* if you're above Top Speed, and with a -1 per full 2 yards/second you are above Top Speed, doesn't seem horrible. I'd only require such every five seconds or so, rather than every second.

*A relevant skill, such as Aerobatics, might work instead.

EDIT: It occurs to me that many characters/creatures may have sufficient stability to be able to maintain control above their nominal Top Speed without a roll. Birds of prey, for example, will frequently dive to build up velocity well in excess of their muscle-powered top speed, then transition into a controlled glide that turns (most of) this into forward velocity. In GURPS terms, that's basically Enhanced Move with a huge Limitation, likely well in excess of -80%. I'd eyeball it as roughly a Perk for each ~x1.5 (x1.5, x2, x3, x4, x6, x8, etc) of Top Speed before control rolls are called for (basically making this a -90% Limitation).