There are guidelines for weapons and armors, but none yet for vehicles that I could find.  To keep things from getting out of control, I thought I'd suggest some, though these are of course just suggestions, as I don't have authority to make these decisions.  Please feel free to use this or ignore this for future decisions.

Vehicles:  
* Movement bonuses are half when in combat.
* A smaller vehicle can be a higher rank for greater stats.
* Small vehicles can only comfortably seat 1-2 average size people.

D-Rank: Small unpowered means of transportation.  This would include bicycles, skateboards, roller blades, small wagons, etc.  
* Up to +100% movement
* No meaningful damage of their own, though you could have a spell that requires this vehicle.

C-Rank: Small powered or medium unpowered.  This would include scooters, go-carts, medium wagons or pulled sleighs.
* Up to +200% movement
* At most D-Rank damage to anyone hit by one of these, and this will probably stop the vehicle too.

B-Rank: Medium Powered or large unpowered.  This would include mid-size cars, small trucks, train cars, trailer homes, etc.
* Up to +300% movement
* At most, C-Rank damage to anyone hit by one of these.
* At most, a C-Rank power is required to stop this vehicle.

A-Rank: Large Powered or small flying vehicle.  This would include a bus, a big truck, small helicopter, etc.
* Up to +400% movement
* At most, B-Rank damage to anyone hit by one of these.
* At most, a B-Rank power is required to stop this vehicle.

S-Rank: Medium flying vehicle.  A medium helicopter or jet
* Up to +600% movement
* At most, A-Rank damage to anyone hit by one of these.
* At most, an A-Rank power is required to stop this vehicle.

I have also considered that there should be something like "combat vehicles" to include things like tanks.  These could have a speed penalty to allow them to do their rank's worth of damage.

Edit1: Not sure if being struck by vehicles should operate like the clash system already in place. It probably should.