MPS, or Maximum Performance Score is a total allowable score per racer for a given race. Each vehicle and power ups will have their own MPS value attached to each item. When joining a race, each player will be able to choose a combination of their vehicles and power ups such that the total MPS of all of those items is less than the allowed limit for that race. The MPS is defined by the race initiator when defining the race.
The MPS rating enables more even competition between players who own different classes of vehicles. With this system its possible for a racecar to compete with a truck. When looking just at the vehicles, clearly a racecar will have a higher MPS as you would expect it to have a higher performance ability in racing when compared to a truck. Having said that, if the truck owner is able to include power ups in the race that can provide him with additional boosts or advantages within the specific race, there are additional strategies that truck driver can apply to level the playing field and compete with a racecar driver.