Some while ago there were some questions asked about changing the gearing on a bike, ostensibly to change cruising speed. The following is specific to an Enfield Electra but is likely to be similar for other bikes.
When I first got the bike I noticed it was not as lively as the old one I had in 1961. Investigation showed that the previous owner had fitted a one tooth bigger gearbox sprocket. No doubt because he had been given old out of date information. Since 1949 Enfield 500's have run with a 17 tooth gearbox sprocket, it became common practice in Europe to add and extra tooth to increase the cruising speed. From 2004 Electras' came with an 18 tooth sprocket as standard as they have many of the learned by experience improvements incorporated in them. This includes a rotary oil pump (which wetsumps), five speed gearbox and a roller big end as well as an aluminium barrel.
My bike with its 19 tooth sprocket returned about 95mpg and would cruise from slightly above 50 to 65 with very little vibration. It did not like to be in top gear below 50 and any incline or headwind would have me shifting into fourth gear.
Generally felt overgeared and under powered.
Now the cruising speed is from 40 to low 60's which is huge jump down the speed range. In good conditions it will even go down to 30mph and up again. I can only say that maybe because the engine is running lighter is can rev a little higher. After a longish recent trip the mpg is still 95mpg but, and it is a big but, it now will handle gradients and headwinds with no trouble at all and i have noticed I am making a significant increase in over the ground speed. On a run of about 65 miles which I often take I have reduced the time by at least ten minutes, even though I don't ride any faster. It is just that I am able to maintain my usual cruising speed of about 55-60 for longer.