depends how much money you have, space and weight requirements, etc...
cvt has frictional losses, not actually that fuel efficient but they do allow you to find the ideal gear ratio needed at the time. the v belt they use always rubs, and thats a loss. plus the belt has to flex...more loss. the only reason they get better economy is that running the engine at the ideal rpm is better, only just outweighs frictional losses.
gears of any type are expensive to cut... but theres nothing to stop you from tearing apart a motorbike gearbox and making a suitable box for it. at which point you may as well use a lifan/pitbike/honda50 type engine, sequential shift is great, can be had in either manual clutch or auto clutch versions. (the auto clutch actually still has a manual clutch, you just hold the gear shift up or down...) lot easier to mount, and spares/hop-ups are cheap and plentiful... can also have electric start. you dont have to make ANYTHING!
the most efficient gearbox would be a chain box...several ratios of chain side by side (not a derailleur like on a pushbike!) with the same dogs as used in a normal gearbox. all you need is the machine tools to assemble such a thing, and as for tensioning...dont worry. online centre distance calculator will let you figure out the best ratios, all on the same centres... dunno if its the same as gears, where as long as the total teeth in any combo are the same, they share the same centre. ie, 32:32 is 64, as is 20:44, 22:42, etc... use HD #35 chain and the whole thing will be pretty compact. seal it up and keep it running in oil, last forever...
and then theres the auto 2 or 3 speed gearboxes as used on the 1:5 rc cars.. work on centrifugal and one-way clutches, shift points are adjusted by spring tensions. once you tear one apart, the concept is fairly simple... double them up and you can have 4, 6 or more shifts without the hassle of manual shifting...
last one...fluid coupling. or torque convertor. pretty bulky but you only need one geartrain, from output to wheel/axle... fluid couplings are used in certain washing machines, always have some "slip" but will smoothly accelerate from standstill to running speed but the torque output will always be less than the input. the torque convertor has a third memeber inside, redirects oil flow that allows for torque doubling... just like you doubled the output reduction when you take off, so fairly similar to a CVT...
there was a go-ped that used a fluid coupling but trying to find one isnt easy...or cheap. it was not a torque convertor (despite the name) just a fluid coupling. and being a few inches across, not as efficient as one would like. torque convertors need to be big to be any good. bigger diameter=faster oil flow= better power transmission.
