Ford and Chevy both have different vehicles and can sometimes outpace the other however after some testing here's what I found out:

Ford:

Modern Muscle: Ford GT Off-road: Ford RS200 Classic Muscle: Ford Roadster Coupe Trucks: Ford F-150 SVT Lightning Racing Cars: Ford Focus Touring Car

Chevy:

Modern Muscle: Chevrolet Corvette Coupe Z06 Off-road: ??? Classic Muscle: Chevrolet Camero Z28 or Chevrolet Corvette Stingray Trucks: ??? Racing Cars: Chevrolet Monte Carlo, Chevrolet Corvette Stingray Race Car or Chevrolet C5R