To get the most speed out of your car you have to get the most horses, however sometimes the transmission will keep you from getting to use all of it. Buy the Full Customization transmission. Now go to the settings either right before a race or at home. Change the settings of the last part of your transmission (the bar that has Sports on the left and Wide on the right) to all the way left. This will move your individual gear ratios. Next, go to the "Final" bar and move it as far right as it will go. Then, go to the next bar up and move it as far left as it will go. Do that to the remaining bars. When racing you will notice that your gears will shift extremely quickly, however, the bad starts will lead to every single horsepower being used. This is really only needed in high-powered cars that exceed 650 horses and should not be used in short races with competitors having equal power to your own.