Right after winning your first race, you'll get enough coins to buy your first car upgrade.
Go to your car garage, select your Dodge Dart GT, click tuning. The cheapest car upgrades you can buy are acceleration and handling (200 coins each).
If you fully upgrade acceleration (buy all its upgrades), you'll get :
If you buy all this car's upgrades, you'll get :
If you do this with two other cars, you'll get :