[ProDigit]

For best mpg, I would recommend a 150 CC, 5 speed transmission,double overdrive motorcycle.
I tried the Roketa 127, best i got is 96mpg at 60mph tailgating trucks, and 120mpg at a constant 35-40mph.
127cc is too small for highways, and made to go 35-45mph, however it's for speed step through gearbox with clutch started slipping, and acceleration was sluggish (a WOT departure was very similar to cars departing from a red light)
Therefor a standard 5 spd gearbox is just the right thing for small bikes of <200cc.
A thumper or single cylinder is most efficient, more so than a twin cylinder, and excellent for <200cc engines.

a single cylinder 150 cc engine, with a 5-speed transmission, & a double overdrive doing roundabout 3000 rpms at 45 miles per hour, will be the closest results you can get to a hundred MPGs, while at the same time, downshifting doing 7-8k rpm allows you to go to about 60 miles per hour at ~70mpg.

Trying to keep the rev range between 2500 and 3500, at your regular cruising speed maxes out mpgs. I usually change +1t front, and -5t rear, or the equivalent of 7 to 8t down on the rear. Most bikes come with a ~16/41t, and mpg is directly affected by sprocket tooth change. Going from a 16/41 to a 17/36t should increase mpg from 50 stock to 60mpg (a change of just under 20%).
Upshifting 1 or 2 gears should give you very similar gear ratio as a stock gearing final gear, so you could do almost the same top speed in 5th or 4th gear.



For prolong highway rides of 65mph tops, you will need A 250 cc engine, single cylinder at 85mpg peak, 75mpg @75mph.

if you ride a lot of highway and interstate, at speeds over 75 miles an hour, you will need a 500 cc engine minimum. a 650-750 cc engine is recommended if you are doing more than one hour at interstate speeds a day.

All of these are best with a ptwin engine, fuel injected, but some modern vtwins also get good gas mileage.

Honda engines are better than suzuki engines. They have lower peak rpm, but higher low end torque, which is excellent for eco riders, and those that want to make their engines last.
For high performance, lower mpg, Kawasaki engines are quite good.

your gas mileage, when fuel injected, can be between 66 and 75 miles per gallon on a 500-750cc engine.
On most carbureted engines it's a good 15% lower