Why is my car on Midnight Club Los Angeles not picking up speed?

I just bought Midnight Club LA on the PSN store last night. When I first started to play it, I had to pick between 3 crappy cars; which is normal. I won the first 2 races, upgraded the car at the chop shop and then turned the game off. When I turned the game back on, and tried to race again, my car wasn't going past 35 mph. Is this suppose to happen in the game? I upgraded it to the most I can possibly upgrade it as far as where I am in the story. I also changed the gaming controls so I can race at my own controls. How do I get my car to pick up speed again? Do I have to get gas or something?