Re: Gearing and Fuel Economy
Exactly... you want to set your cruise rpm so the engine has as little load on it as possible but at the lowest rpm you can run it without it loading up.
going fast on a trike can be dangerous because of their inherent tendency to roll over in a turn if you try to turn going too fast. You can do things to lower your center of gravity to help as well as widening the track on the rear wheels but this will just raise the speed in which it'll lose stability. Try to mount your engine as low in the frame as possible and set the seat bight as low as you can ride it comfortably... this will help some, and every little bit helps here.
Personally I wouldn't ride on a trike faster than 25 or 30 mph unless on a long straightaway and slow way down at the turns until you really get used to how it rides and where its going to start to tip over. You can do this in an empty parking lot or on a wide enough road to find what speed one of the rear wheels will lift off the road and get a feel for it so you can recover before it tips over in case you get in a situation where you may need to swerve to avoid a crash.
Back to the gearing, if running a China girl engine you can reach up to about 40 on a trike with 26" wheels with a strong engine and a 36 tooth rear sprocket but acceleration might be weak. You can hit 40 with a 44 tooth but you need to port the cylinder so it can hit 9500+ rpm. A 40 or 42 tooth rear would probably give the best all around performance with a decent cruise speed at around 30 to 35 mph.
If you're going with a 4 stroke or andifferent 2 stroke let us know which engine you have in mind so we can figure the best gearing. My opinion gonna 4 stroke that would do good in a trike would be a.predator 212 with a Comet CVT and a 50 tooth rear, the cvt would let the bike accelerate very well but the rpms will drop considerably at cruise since these will sense engine load and rpm to set the ratio right where its needed at any rpm or speed. I had a go mart with one of these torque converters whenni was younger and it accelerated really well and topped out around 60 mph but would cruise all day at 30 to 40 without feeling likenit was over reving all the time, then when ya gunned the throttle it would take off good.