Umm...
Do you mean totally new mohicles, or recreations of existing or historic vehicles?
For the former, just think of a model vehicle, and try to figure out what it would look like in game, and tweak that (what? a motorcycle that looks like a Fatboy but is much faster? OK).
For historical vehicles, I'd just find some info like size, speed, armament, etc. and try to translate those. I've been using the vehicle rules (slightly modified) from "Thunderball Rally" (Polyhedron 152) to emulate some WW2 planes.
I give you... The P-51 Mustang!
Class: Prop (Air)
Crew: Skilled +4 (1 pilot)
Size: Gargantuan
Initiative: +0 (-4 size, +4 crew)
Passengers: 0
Maneuver: +0 (-4 size, +4 crew)
Cargo Capacity: Special
Defense: 13 (-4 size, +7 armor)
Speed: 965 ft
Maximum Speed: 437 mph
Hull Points: 85
DR: 14
Cost: Be a government first
Weight: 9,895 lbs
Ceiling: 42,000 ft
Range: 450 miles
Armaments: 6 .50 machine guns, in pairs
Ammo: 2 inner pairs, 400 each; outer pair, 270 each
Atk: +4 ranged (-4 size, +4 crew, +4 equipment)
Damage: 4d10 (pair), 5d10 (any four), 5d10+5 (all guns)
Range: 200 ft
Critical: 20
The P-51 Mustang is a plane with wonderful visibility from the cockpit, excellent range, and a large engine and bountiful weapons. It is vulnerable to damage from below because its radiators and heat sinks sit proud of its belly; any critical is assumed to have damaged these pieces.
External Stores: The Mustang can pack a pair of fuel tanks under its wings, which extend its maximum range from 450 to 750 miles, while reducing its Maneuver rating by -2. These drop tanks can be released as a free action.
Cheers! And remember, this is only v1.0.
TWK