Just hopeing someone could give me some info on what size sprocket to use.
I have a 79 kz650 that runs 5000 rpm at 100kh and i would like to be runing around 3500-4000 rpm at 100km

Try droping it 6 teeth on the back. Also check on the size of the front sprocket, someone might of changed it also. You can go thru mathimatical calculations but I have had several 550's and they really benifited from changing the gearing. Just be ready to shift down a gear or two to make a quick pass.

If you do 5000 rpm at 100Km/h, and want to drop revs, then consider you can only drop a maximum 10% before the bike begins to lose out on you. You may think she will do less rpm, hence less engine fatigue and less fuel consumption, but this may not be the case, actually it may be the other way around.

Less rpm means you will need more speed to reach the same rpm before shifting one gear up, which may mean troublesome driving in hilly country, mountains or even chaotic city driving. Also, you will need to keep the throttle a little more open.

I discovered this when moving from summer tires to winter tires on my car, the winter tires are a little narrower and about 3% smaller rolling radius, so in theory I lose a little top speed. Well, mileage has improved, I can now do my full week's commuting (700Km) with one tankful when I previously needed to top up on the last day.

I suggest to increase just one tooth on the front sprocket; it beats having to work on the rear wheel, it allows to carry one (or two) spare sprockets in your tool roll and only needs limited adjustment on the chain tensioner. i.e., if the standard sprocket is 20 teeth, buy both a 21 for high speed cruising and 19 for mountain driving, and try them out.

Maurizio from Rome, Italy

[This message has been edited by mpescatori (edited 21 February 2006).]