Dubepj, there's just no easy answer to your question. How much vibration the motor produces depends on state of tune, crank runout, and balance factors. Some motors are pretty smooth at 7K rpm and some shake like a dog passing peach pits north of 4500. Then there's the matter of proper torque settings on motor mounts, rider gear (how well boots and gloves isolate vibration), and, last but certainly not least, rider tolerance. But it's always better not to work the motor any harder than you have to. If the bike were mine I'd install a 34 tooth rear sprocket; I found the 17F-34R combination to work quite well on gravel roads back in the day before our local politicians started harvesting contributions by passing work to their contractor pals to pave everything that couldn't get up and run.