[tshiggins]

I'd agree with most of what everybody has said, thus far, and have given some thought to this, myself.

What matters in displaying this on a map is the relative velocities of the vehicles involved, and not how fast each of them moves across the landscape. So, when you show it on a battlemat, just figure out how relative velocities and relative distances change in relation to the PCs' vehicles.

Now, you do need to throw up some road hazards and obstacles and such, but those would move from one side of the map to the other, rapidly, as the vehicles remain more or less centered.

Also, bumping and jumping and rattling around would make combat of all sorts more difficult, and I'd require a DX roll with more or less penalities for an individual to move more than a single hex, inside or outside a vehicle.

(Crawling out onto the hood of a fast-moving vehicle while somebody is shooting at you, to spit fuel into a ram-scoop intake for a turbocharger, is completely insane and penalties to DX rolls should reflect that.)