help help
 

i am thinking of going down the bike engine root. iam just wondering what is the best diff too use and ratio too. fireblade/r1 engine.

Thats a bit like asking how long a piece of string is. What will the car be used for 80% of the time? Is it going to be a trackday car or to be used on the road?

road use all of the time

Depends what u want really. 3.92 mental acceleration but low top end and reving its nuts off on the motorway.
Personally had a 3.62 with a zx9 motor and was still quick as f*$k but much nicer to drive everyday.
Other option is buy a 3.92 and try it and if u dont like it buy a 3.62 as its an easy swop .:D
( a piece of string is double its length from the centre !!!!!!!!!!!!)

One of the guys I rent a barn with is doing an R1 Locost, all the calculations say 3.38 ideal, 3.6 max's out at 110 and you spend all the time with a screaming engine.
3.9 would max out at 95 and gear changes would frantic