It’s a novel. You can make up anything you want. You can have a spaceship from plaxonia come down and zap him with a super cosmonium ray.
The simplest answer is that the witness was lying about the speed. Maybe a co conspirator.
Quoting Yosemite "How about this;
As he descends the hill the truck following him loses it’s brakes. He gets rear ended by the truck and his rental 1966 corvair bursts into flames just as he enters the curve."
The truck can be driven by the killer. S/he can hit the Pontiac from behind with a right to left, or left to right motion, depending on which way it needs to go over the cliff. The brakes don’t need to fail. If there are no witnesses, s/he can simply drive away. The killer needs to be sure it’s the right car before s/he hits it. If it says Corvair, it’s the wrong one.
Note: Cars do not burst into flames until they go over a cliff. Haven’t you ever seen a movie? They ALL burst into flames, often in mid-air, once they clear the road.
This was part of the plot several times in the 1950’s tv series “The Adventures of Superman”. The bad guys usually used acid on the steering knuckle, with the idea it would steer ok at first, but the acid would slowly eat through the metal and eventually, no steering. And over the cliff Lois and Jimmy go, plummeting toward a certain death. But of course this is no problem for Superman, who’s already on the case and catches the car before it hits the ground.
As I read your posting the first thing I thought of was the famous college runner Steve Prefontaine’s fatal car accident in Eugene, Oregon. You might Google that and get some ideas too.
Your post also reminded me of the accident which injured the F1 race car driver, forget his name at the moment, but it was portrayed nicely in Ron Howard’s movie “Rush”.
yeah I like the idea of keeping it simple and realistic like the truck rear ending the car, sending it over the cliff. I’m still trying to figure out how a car magically got back to town again in Lee Child’s first Reacher novel.
Has anyone besides me considered the possibility that the OP might have something more devious than writing a novel in mind ?
No.