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The economy isn't going to be much better than operating the motorcycle itself, if it is as good. Most 400cc motorcycle twin cylinder bikes don't get 80 mpg ridden normally. Singles tend to do a little better than twins, but once again, 80 mpg is rare.
They are set up to perform well, not to deliver stellar economy. The reason they can get very good economy is, they don't weigh very much, and there isn't the wind resistance that a car has to deal with. Ridden carefully, a motorcycle can get very good economy, and the engine is unlikely to be damaged unless it is lugged excessively.
The cam profiles used in motorcycle engines are not going to be ideal for this sort of application. Changing the profile will result in high cylinder pressures, and the timing curve would need to be modified. Most of the 400cc twins I know of were carbureted. Those carburetors were mostly CV's. Constant Velocity carbs will altitude compensate and operate close to stoich until excessive loads are placed on the engine at low rpm. Low air volume can create a lean condition, and the carburetor will have to be jetted up.
Lack of a reverse gear is one thing to keep in mind. Another would be continuous operation of the chain to the output shaft. That will create drag. Installing an over-run clutch at the rear adds weight and complexity.
Generally, motorcycle engines are not built as heavily as automotive engines. The life span of a motorcycle engine is lesser because of this. Air cooled twins are generally in need of rebuilding around 20k miles. Water cooled engines can double or triple that with appropriate maintenance. Most often it is cam drive failure that does them in, though due to the performance nature of the build, most will suffer excessive wear sometime over that 20k mile number. Everything is lighter to allow for high rpm, and therefore high specific output.
Look at it from a HP per Litre standpoint. A 25 hp 400cc engine makes 62.5 hp per litre. That's a pretty old engine. I have an '85 250cc engine that makes 45hp in stock configuration, and gets about 36mpg. That's 180 HP/L. In racing trim, that same 250 makes 66HP, or 264 hp per litre. In that trim it gets about 12mpg. Pretty extreme, but hopefully you'll see where this is going. (The record for a naturally aspirated recip is 400HP/L for those in the need of such trivia.)
Motorcycles of today generally don't get very good economy. 600 class machines get in the 44-46 mpg range. (Four cylinders.) They are set up with extremely radical cam timing, and make anywhere from 100 to 120 HP. Let's say 200 HP/L. They don't make very good torque down low. That's going to be important when getting a car moving.
Twins and singles do a bit better, and make better bottom end. Still not anywhere near what has been suggested (80 mpg) until you dip down to the 250cc and smaller machines.
A 1 litre Metro XFI makes 47ish HP. That's 47 HP/L
A base metro makes 55hp. 55HP/L.
These cars get pretty darn good economy. Better if driven wisely.
To cruise at 25 HP, the 400cc engine would have to be near the top of it's HP curve. Screaming.
To cruise at 25 HP the Metro engine would be near it's ideal torque peak. Loafing.
Motorcycle engines are not designed to pull a lot of weight around. The cooling systems are inadequate for doing lot's of heavy work, like accelerating a car. (Unless you look at some of the exceptionally large ones like a goldwing engine.) Even so, the cooling system is not designed to work under heavy loads at low rpm. Unbeknownst to most folks, most motorcycles will build heat when operated in stock trim at speeds somewhere around 100 mph or faster. They will build heat and be unable to keep even with that build up. The coolant system itself is not the only factor in keeping things cool. Incoming air charge, fuel, oil capacity exhaust scavenging and so forth are factored in. When operated outside design there are a lot of unknowns. The speed at which a sub-litre bike can't cool is much lower than 100.
A Honda 600 got about 60 MPG. It had an air cooled 600cc two cylinder that was roughly similar to twin cylinder motorcycle engines of that era. They didn't live very long.
The domestic market and europe got the Honda 360. It got better mileage, but still not 80 mpg. It could be done, but not under normal circumstances.
I was in the motorcycle industry through the 70's up into the early 80's. I still keep tabs from time to time. HP has gone up, and the technologies are remarkable. These engines are not ideal for automotive applications though.
With modification, a motorcycle engine could be made to do the work suggested, but the cooling system, and longevity of the engine would still need to be considered.
Just a few thoughts before you get set on a path.
CJ
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