Has anyone changed the gearing?

[ChipBeck_imp]

As soon as I get my 19/35 combo from 7GD, I'll be posting my results. I don't expect any problem off the line, only a great reduction at hwy speeds 4,000 will become 3,401. The 14 models do 3300 @ 70. I also expect hwy mpg (@70mph) to go from 43.5 to apx 50. The 3.9G tank will then be matched to performance correctly - IMHO without the need for a 4.4G tank upgrade.

It's a common misunderstanding, that lower revs would mean better mpg in every case.

A petrol engine has its best fuel economy at revs at max. torque, which is 5,500 rpm for our CB.

That means, doing exactly the same speed in the same gear at 3,300 instead of 4,000 will reduce mpg. But luckily not much or noticable, as our motor has a very flat torque characteristic.

For the details: https://en.wikipedia.org/wiki/Engine_eff...29_engines

Cited: "Engine efficiency peaks in most applications at around 75% of rated engine power, which is also the range of greatest engine torque (e.g. in most modern passenger automobile engines with a redline of about 6,000 RPM, maximum torque is obtained at about 4,500 RPM, and maximum engine power is obtained at about 6,000 RPM). At all other combinations of engine speed and torque, the thermal efficiency is less than this maximum."

Django,

The "engine efficiency" referred to above is power production, not fuel mileage. By definition an engine operating AT FULL THROTTLE is most efficient in power production when it is able to make the most power PER RPM. (1 power stroke). Torque is turning force PER REVOLUTION. Horsepower is that torque times the number of revolutions per minute. None of this has much to do with fuel economy especially when operating at part throttle as a car or bike operates 99% of the time. Pumping and internal frictional losses are huge power consumers and they increase in a pretty linear fashion as rpm increases. This is why automakers gear cars to operate at often less than half the rpm where peak torque occurs. When operating at partial throttle (and nobody cruises at full throttle) the lowest rpm that will generate enough power to keep a car or bike at speed, in other words, enough power to overcome frictional losses, pumping losses, and wind resistance, will yield the best MPG as the first two of those three components are directly related to RPM.

I was both a domestic and import car dealer for 25 years and I'm a graduate of the General Motors Institute in Michigan where we studied auto and engine design as well as marketing. The efficiency cited in your example is efficiency to both inhale and completely burn the maximum amount of fuel and air at full throttle. This efficency starts rising above idle and keeps rising until RPM makes intake and exhaust openings too short to allow enough airflow and then it starts to fall off. But again, this is only a measure of torque at full throttle to measure maximum possible power (this is why all dyno runs are made at full throttle) and this is meaningless when it comes to measuring mileage while cruising at 20% throttle.

The bottom line is that my CB1100 gets 3 more MPG at 70 MPH turning 3600 RPM with my new gears than it got turning 4000 RPM with the stock gears. Cheers.

Chip