With the stock 140/80 rear tire at 60 mph, the 4 speed bike will be turning 4,135 rpm - while the 5 speed bike will be turning 4,008 rpm. That is only a 127 rpm difference.
If you were to change to a 140/90 rear tire at the same 60 mph the 4 speed bike will be turning 3,952 rpm, the 5 speed bike will be turning 3,831 rpm.
Adding a Kawasaki front pulley (25 tooth instead of 23 tooth) will drop the rpm 8%. In combination with the larger tire you could get the rpm down to 3,636 for the 4 speed and 3,524 for the 5 speed bike.
So.....if lower rpm is your goal - the cheapest and easiest thing to do is to install a 140/90-15 rear tire and do the Kawasaki front pulley. If you have a tired 4 speed bike and need to do the engine work anyhow......find a good 5 speed bottom end.
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