49cc HS or 79 predator gearing


New Member
Jun 8, 2017
hi, question about 4 stroke builds,, I'm looking into what ratio I'd need for otherwise stock HS or a small predator engine. 27.5" wheels MTB w/ 2" road tires and get a top speed of only 28-30mph. thats plenty fast for me lol. I like torque so really would like to build a 79cc predator and not rev it out just to get up to a good speed.
I expect some hilly travels, not much tho mostly flat. should I get a j-shaft kit with the predator? what kind of ratio should I run on either?
thanks for helping on my next build! planning phase just right now


May 25, 2008
Howdy Chmn. I was avoiding answering as my math skills are notoriously bad and have not had ever, never used 27.5 inch wheels. (but as soon as I say something dumb, folks will help, lol) But hate when I ask a question and folks don't respond.

More out of habit, I aim for 12:1

Using the calc with 27.5 wheels @ 12:1 and at 5,000 rpm. I get a top flat speed of 34.1 MPH.

Never trust my math! but if you were using the average off the shelf jackshaft w/ 11T clutch, x22T, x9T 54T wheel sprocket... yada yada, it works out to that at 5K RPMs.

This is the calc I used; http://jimsitton.net/ratiocalc/ But honestly, on this stuff triple check my thunks. That calculator is purty awesome and has never failed me.

Build something cool!