to change the speed or mass or what ever about the car, go to c:/program files/rockstar games/grand theft auto vice city/data/handling.cfg
now before you begin to do anything, make a backup copy of the handling.cfg file so if you mess up and everytime you go to the game and find the car that you just modded up in the air and exploding every 5 minutes, you don't need to memorize much =p
now pick a car that you want to mod and you will notice a line of numbers next to the vehicles name. now it looks confusing now but its not. scroll up a little bit and they give you a key to what the numbers mean, awesome.
you can either manually change the numbers to your desire but they have to be within the keys numbers or the game will shutdown everytime you get in the car.
or...you can copy the characteristics of another car to that vehicle that you are modding. this will keep the same skin, the same sound, and the same sound effects but will change everything else, i.e. speed, accerleration, deaccerleration, turning radius and so on.
and thats how simple it is =)
btw drumthrasher, its pirated fraps i'm using to capture the gameplay.