todd

Good question. But apparently thats what it is.
-On Halo2 if you break away the edges of the hood, you can see headers, valve covers, an intake, air cleaner, and what appears to be a muffler (or cat. converter perhaps), meaning it's some sort of internal combustion engine. Not to mention the sound it makes when at idle is obviously made by a combustion engine.
-By listening to the sound the engine makes as you accelerate, if you listen closely you can pick out when it changes gears several times.
-If I remember correctly, an article on Bungie.net said it was powered by electric engines in each of the pods that the wheels are connected to.

I don't know much at all about electric engines or how they work in conjunction with gas engines (hybrid), but maybe the transmission allows differant levels of power to be sent to the electric engines?

Or perhaps the 'transmission' is actually just a few gears attached to each of the electric motors in the pods, rather than the gas engine.

Or maybe the sound that sounds like gears changing isn't actually what it was meant to be? Maybe that was just a goof up and there is actually only 1 speed. The Warthog in the first Halo only had 1 speed, so that would probably make more sense. I'll study the sound again later and see if I can figure out what else it might be.