As has been said, even at say 25, it's going to take some distance to stop your vehicle. My truck is no 'Vette, so I'd gun it and get some distance between me and the gunfire. Pull off the road and lose some elevation, if possible. Grab the cell and call 911 giving them the best description possible. AS soon as possible, I'd be reaching for my Kel Tec SUB2K and getting it unfolded and ready to return fire. If time permitted, I"d get in the back seat and grab my little chest rig that's set up with six 15 round .40 cal mags. Depending on the situation and available cover, I'd try and eek closer to observe/help out. I carry a small pair of binocs in my truck for looking at wild game, but they might come in handy for a situation like this.
I'd like to say I'd stomp the gas and ram the guy in the flannel. But at midnight at 25mph, how can you really be sure, dead sure, who's the bad guy and who's the good guy. yes, most of us want to do the right thing and be a hero, but what if there's some odd situation and it's an off duty cop pulling someone over for reasons we don't know. If you can positively ID who's who, it's a different story. Either way, I doubt your insurance will pay for you totalling your car by using it as a weapon and if you think a podunk town has money to buy you a new truck because you were a good samaritan, well, I bet you're wrong!
Doug K
That would be the last thing on my mind.