A jet propelled car would be a lot of fun for speed runs! I'm more of an electric guy, but I thought about making a car or airboat using an EDF jet engine. The Dumas Swamp Buggies are too cool, but from what I hear even a brushless motor is too weak for it. I was gonna try to use an EDF fan instead. If you apply that same thinking to a car, you get a rocket car!
You know, though, since we're getting crazy here, I could always attach wheels to the airboat and make an amphibious vehicle.
I actually just picked up a Tyco Terrain Twister for $20 on Craigslist.
It's a goofy looking amphibious vehicle that uses twin augers, kinda like an Archimedes screw, to propel itself. On dirt, it's slow and hard to control forward, but it scoots when you go sideways. When moving sideways, you can shift the weight of the vehicle to turn. In the water, it's not the fastest thing in the world, but it drives effortlessly. I convinced a buddy to buy one, too, and we have a blast when they flood the park. It's a nice change of pace, and we can't help but laugh when we run them. We can't wait to try them out in the snow or some soft sand!