Sorry for necro, hadn't seen response. Still relevant I think.
Right now, we're relying on the physics engine to handle that stuff. It can be wonky, and award damage to the lighter/slower vehicle. Not sure we can fix this, but we might be able to try a work-around. Worth considering.
Also, there should be a monetary discouragement for ramming. Bringing a fresh vehicle onto the battlefield and ramming with it should be detrimental points/money wise, while ramming as a last-ditch effort should pay off in some cases. Economy and pricing balance remain WiP.