Incorrectly regarded as goof: Character error, NOS
This may be incorrectly regarded as a goof:
Character error
Whenever one of the characters hits their NOS buttons, there's always a sudden look of surprise on their face as if they didn't expect their car to go faster...
Often a NOS (Nitrous Oxide System) may be a last resort, a "ace up the sleeve," if you will. This means it is not normally used unless you really need it to win. The characters are cocky, and easily convinced to bet a lot of money. They probably defeated lesser, amateur racers with ease. This would explain the surprise.
A Nitrous system is capable of having the jets that (known as 75 shot, or 100 shot IE +100 Horsepower). If the character had their NOS jets changed and upgraded, they may be surprised at the additional sudden power.
These systems can cause major damage and thousands of dollars of engine and drive train damage if installed and used incorrectly. They probably were installed and tested by a shop/tuner and now are being used by the "surprised" racer real world.