This is what I came up with:
Rating Examples
0 Cog, Heavy Barge, Freighters
1 Barge, Aircraft Carriers
2 Rowboat, Caravel, Fishing Boat
3 Galleon, Cruisers
4 Trireme, Longship, Frigates, Destroyer
5 Bus, Armored Truck, Tractor Trailer, Sailing Coastal, Sailing Corvette, Sloop
6 Limousine, Armored Truck, Tank, Forklift, Naval Corvette, Hydrofoil
7 Snowmobile, Compact Car, Jeep, Pickup, Antique Car, Formula 1 Car, Patrol Boat, Littoral Combat Vessel
8 Sedan
9 ATV, Dune Buggy, Speed Boat
10 Sports Car, Police Cruiser
11 Dirt Bike, Jet Ski
12 Racing Bike