One thing that gets me about the movie the more I think about it is the opening scene to the first movie, where Frank refuses to budge because there's too many people in the car and therefore too much weight. He states that with the added weight, he doesn't have enough fuel in the car to make the getaway. Now, I understand that it serves as an Establishing Character Moment, to show he's fastidious and detests any disruption in his orderly life. But in-universe, it doesn't seem to make any sense to me. How can be possibly have planned every swerve and dodge he may have to perform during the getaway, so that he knows the exact amount of fuel he will need to lose the police and get home safely? It seems that if he's as prudent a planner as the movie indicates he is, he would have allowed for extra gas in his car to prepare for inevitable deviations from his plan. Or just filled the damn tank.
A deal is a deal. If you hired him to bring you and two of your friends from place A to place B at a certain time, then you hired him to bring you and two of your friends from place A to place B at a certain time. He has to be sure that everything is perfect for him to do the perfect job at every point, because he has to make sure his reputation is flawless.
It's also not just the fuel, but also the shock absorbers he has installed on the car: the extra weight counters their effect, which means the car would be likelier to break down or have mechanical problems or not turn a corner fast enough.
The fuel was simply another way of making Frank's point: any alteration to the deal could cause the plan to go FUBAR. Being as Crazy-Prepared as he is, it's unlikely Frank literally didn't have enough fuel for the extra person; he was just proving a point.
