1000HP V-16 running on regular gas? I find that hard to fathom. Displacement must be freakin enormous, and compression must be nill.

The number of gears does matter to some extent, although it depends depending how much torque it has. To save fuel on the highway, especially given 16 pistons going up and down, a 5th gear would seem needed.

But given it will probably make at LEAST 750 lb.-ft. of torque, you can make 4th gear as long as hell and lug at 1800RPM at 70mph all day long.