Hmmm... let's see... there are 7000 grains in one pound; a 230gr .45 bullet is about 1/30 of a lb... At 900fps call it 30lbs-ft of momentum (roughly).
My Avalanche is around 6000lbs. At 30mph, that's about 180,000lbs-ft of momentum (roughly).
Net momentum is 179,970lbs-ft of momentum, still coming right at the shooter....
But let's say I have a .30-06... 180gr is a bit over 1/40 of a lb, times 2800fps is a bit over 70lbs-ft momentum.
So when it hits the truck, the truck now only carries a net 179,930lbs momentum at the shooter...
As others have noted, getting out of the way is critical. Shooting is pointless, if one does not get out of the way.
From a legal standpoint, shooting may or may not be defensible. After you are out of the way, from a legal standpoint, you should probably try to determine if the guy is really trying to hit you; if he's drunk or otherwise intoxicated and simply incapacitated (and you happened to be in the wrong spot); if he's having a diabetic event (LEOs sometimes find their DUI suspects are actually not drunk at all, but suffering from sudden, acute blood sugar drops); or if some other thing is at play.
In most cases, if the driver isn't somebody you know to be an enemy, it's probably one of the other cases. Or he's a wack job.
Safest thing to do, legally speaking and once you are out of the way of the vehicle, is to get to someplace safe. (Large boulders, concrete barriers, ditches, etc that will stop the vehicle from following are all good things to use, if available.)