The buggy diffs by hb are 3.31 ratio.
The truggy diffs by hb are 4.3 ratio.
The weight of the car and how many cells depends on how fast you can go as well. The quality of batteries. You also have to figure in the amount of drag and stuff different buggies and trucks or even cars have. 70 MPH in a mt is harder to do than a street car.

I would say very hard to do in an MT, but a street car? I'd love to see it, it would be crazy to control though! If your looking to hit 70 with a BL, it's gonna take money because you'd want to build it right, also, dump the mt and build something that can help control the speed, you hardly keep a mt straight at 60, and that's if you can find a way to put the power to the ground.