Hey guys, I've got a question to bother you with!

So my boyfriend and I are going to be picking up a couple 1970's DT-100's in Montana this summer and going on a little road trip with them. My dad is insisting that he knows EVERYTHING there is to know about DT's since he had one 26+ years ago. He says that you can make it go faster by adding a gear to the back or front..(whatever that means, I'm not a mechanic but my boyfriend is) and it will increase the top speed....What do you think about this?