Ive been playing with my fazer 600 2001 sprocketts and went from 15/48 to 17/45 now trying 17/42 - sitting behind a lorry commuting all motorway at around 56 im averaging around 76 mpg which at 140.0p per litre saves me around two pound a day sat at 70 mph. im hoping this new sprockett will give me more.
the only snag is sitting at that speed but what the hell im saving money and when i do get to work my lorry has been limited to 52 mph so im kind of used to slowww.
check this out -
Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator
very helpful site