I saw the BEC gearing / diff calculator but from some kind of reason I can't make it work on my computer ...

If anyone can do the calculation for a sec and let me know what I end up with that'll be greatly appreciated.

Using 5VY R1 motor, the gear ratios are -

Primary Reduction Ratio 65/43 (1.512)
Secondary Reduction Ratio 45/17 (2.647)
Gear Ratio - 1st Gear 38/15 (2.533)
Gear Ratio - 2nd Gear 33/16 (2.063)
Gear Ratio - 3rd Gear 37/21 (1.762)
Gear Ratio - 4th Gear 35/23 (1.522)
Gear Ratio - 5th Gear 30/22 (1.364)
Gear Ratio - 6th Gear 33/26 (1.269)

I'm running tires with OD of 20.5".

My diff ratio is 3.636" but I'd also want to know what diff ratio I'll need to get to 140-145 MPH.

Thanks!

Moti

[Edited on 13/2/09 by Blackbird]

You'll need another 100 or so bhp to get to 145mph, imho of course.

I'd be more inclined to work to a more realistic 120 or so.

ATB

Simon

Let's just say 140 mph in theory than.

Moti

I have some spreadsheets on my website for gear ratios for popular bike engines. Linky
They are designed for chain drive but you can substitute the front and rear sprockets with the number of teeth on your differential pinion and crownwheel.
There are a couple of sheets for the Yamaha R1.
As other posters have implied you are looking for a final drive about 3:1 to achieve your top speed (assuming you have sufficient power).