Upgrade your first buggy car only a few times. You will soon move on to the next vehicle after collecting 15 diamonds. Spend more coins upgrading the second buggy, which has better stats. Spend most of your coins upgrading the Monster Truck. You will be using it for awhile, as the next car in that category will cost 80 diamonds which will take some time to earn. Consider upgrading Speed and Grip first because they will help you win races. Also increase Acceleration when possible, especially with the Monster Truck.