I think i've finaly made up my mind about engine choice, think i'm going to join the crowd and go R1

so what diff ratio is recomended and what top speed does that equate to

thanks

it depends on your R1 and your intended use.

I have a carbed R1 with an 11750 RPM red line. With a 3.62 diff and 195/50/15 or 205/60/13 tyres that gives 120 MPH max. It is fine for the road but acceleration drops right off above 105 on the track.

A 3.38 or 3.14 diff will give higher top speeds at the expense of less acceleration in 1st. Revs will be lower on the motorway too.

Later R1s rev to 13750, so may have a higher top speed, but the gearing is closer together, so maybe not.