If you really wanted to get finicky with final ratios on the 5 speeds you could fit an 18/19 tooth front and then also fit a rear drum with extra teeth (you can get more teeth just cannot go smaller) to fine tune it.Lot of mucking about for minimal benefit though.
Just put an 18 tooth sprocket on it. That would have the same effect.Bare
I don't want to go much faster than 70-75, but I'd like to do it at lower revs.
I think you have missed the point. It's not higher top speed that we are worried about. It's lower high speed cruising revs. I'd rather not have my engine screaming bloody murder at me if I have to do 70 for a stretch.
Ok. You have three choices, a bigger drive sprocket, A smaller rear sprocket, or a larger tire.As was mentioned, the way these gearboxes are engineered, top gear will always be 1/1 regardless of how many lower gear ratios you have. It would take a total redesign to have overdrive gears in this box. It wouldn't even be this box. the reason for this design is that in top gear the gearbox isn't using any gears, so is more efficient.Bare