The number of gates fluctuates. I think that by the time the 46 seconds mission comes up, the gate count can be as low as 18. I squeaked by at 45.95 seconds.
I wrote a solution
on TA. Here's my 2 cents about the 46 seconds. Maybe the trick for Max Speed will help also.
-Complete a race in 46 seconds
(Probably the hardest mission for most people.)
The number of gates fluctuates at the high levels. I think the lowest is 18, but it can go as high as 26 or more. Wait for a low number of gates, miss none of them, make a perfect start, make every jump perfectly, and tuck as much as possible. You'll still need some luck in landing after the jumps, since that slows you down the most, even with a "Perfect Jump." After several attempts, I got 45.95 seconds. (Missing a gate is a 2 second penalty, making 46 seconds impossible.) I'm afraid I don't have a sure-fire trick for this. Just practice and don't give up.
-Reach max speed
This was my last mission. I ignored the gates. I couldn't seem to get over 140 km/h until I crouched all the way to the ground. My fists were touching the ground. When I got to 146 km/h, the mission was complete. Maybe tucking this much would have helped with the 46 seconds, but it would make turning very difficult.