Terrible. The 40 mph governs how far out in the canyon he will travel over the time taken to fall 600 feet.
The time required to drop 600 ft. is driven by gravity, coupled with the initial downward speed of the car.
From your freshman physics class, you will recall that
d = d0 + v0t + 1/2 agrav t2
Since he drove off the edge horizontally, his initial downward speed v0 is approximately zero, as is his initial distance, d0.
Thus,
d = 1/2 agrav t2
d = 600 ft, and agrav = 32.174 ft/sec2.
So we can solve for t to get a "hang time" of about 6.1 seconds.
Over that time he would travel approximately 360 feet out from the edge of the canyon.
lmao