The spreadsheets Teamxbox has put together will provide valuable performance data on each car. Cars are put into classes based on five parameters:
Speed - This is about raw horsepower. A car with large displacement or running big boost will excel in this area.
Acceleration - This has less to do with horsepower and more to do with how effectively the power gets to the pavement and vehicle weight.
Braking - This is tied heavily into vehicle weight for the lower classes, and is more about large rotors, race compound pads and heat dissipation in the higher classes.
Handling - Things such as center of gravity, weight, drivetrain layout, spring stiffness and tire choice play a large role in this statistic.
Rarity - This number represents the likelihood of ever seeing one of these machines. Each car has a general rarity (Regional Rarity is for the Career Mode).
Each of the above car parameters is based on a scale from one to ten, with ten being the highest. Cells highlighted in yellow show the highest numbers in a particular class, which gives a good glimpse at the top performers, and those cars that are on the brink of heading into another class.
If you pay attention to the data below while reviewing the accompanying screenshots, you'll see that each vehicle has a number somewhere below 1000 associated with it. This is how well the car stats up in the class, and is directly related to each vehicles core statistics, plus any mods that you do on top of the base vehicle. Once a car is modified beyond the 1000 point mark, it jumps up in class.