[Phantasm]

I'm not sure how Car Wars scales its HP. However, vehicles and other machines in GURPS have an equation in the back of Basic Set: Campaigns which if I'm not mistaken is 4x the cube root of the mass/weight in pounds. I'd figure the Car Wars and GURPS damages proportionately; if an attack in CW reduces the car's HP by 20%, take 20% off the GURPS HP and vice verse. A car's mounted weapon against infantry I'd probably use an equivalent GURPS weapon's damage; if it's a 5.56mm NATO machine gun on the car, I'd have the gun doing 5d pi against infantry, like the M249 SAW. A car mounting an M60 GPMG would do 7d pi to infantry. Infantry firing their M16-esque weapons at the cars would have to deal with the cars' GURPS DR and HP.

YMMV, of course, but that's how I'd start working things.

(Then again, I'm a nutcase that once ran a game with players using different systems: D20 Modern, GURPS, Palladium, and Shadowrun 3e all at once. 'Twas interesting when it came to handling initiative, and cross-system weapon damages.)