Since there isn't a penalty (you don't lose any fuel) for restarting during the race, feel free to restart if you're positive yo won't meet the goal you were working for. This way you'll only use fuel when you successd rather than wasting fuel during a failed attempt.
Since bike upgrades can be expensive, save cash by finding certain parts before you commit. When looking around the map and considering the races you accept, it is wise to preview the potential rewards ahead of time. Select the missions where there is a chance that you'll spin the wheel and earn discounts, or you'll wind up spending much more than necessary.