I haven't got any specific fuel consumption numbers for you. Don't know. Most motorcyclists don't need that information. It wasn't until the last year or so that manufacturers bothered to list fuel economy ratings, and that is spotty at best.
As far as size, that isn't what I was trying to explain. Motorcycle engines are "Overbuilt" to a normal operation standard ~20%. They will take 20% more load than normal riding places on them without breaking. They are also generally rated for intermittent operation at high power settings.
An automotive engine is "Overbuilt" to anywhere from 200 to 300%. It will take up to 3x the loading that would be considered normal operation. The parts are bigger and heavier to do this.
This is easiest to visualize where we have a motorcycle that operates at peak power and torque at greater than 8000 rpm. Most car engines peak somewhere around 5000 rpm. Some lower, some higher.
The weight of the reciprocating assembly is made very light to allow for these engine speeds. The crank shafts use roller bearings. Low drag, and highly susceptible to shock damage. The engine is designed to accelerate very quickly. Much more so than an automotive engine, running plain bearings, and with much heavier rods, pistons and so forth.
Theoretically the lighter engine should operate more efficiently at a given engine speed, It also means that there is less mass in the crank and flywheel to provide torque in between power pulses. A two cylinder vertical twin rotates 360° between power impulses. That's where pumping losses start to become noticeable as well. A four cylinder engine provides power impulses 180° apart. An 8 cylinder rotates 90° between power pulses. That's why they run smoother as well.
Once up to speed, operating somewhere near torque peak, your application could very likely get better economy than the 1.6L Honda engine that came out of the Civic. All things being equal as far as acceleration and load, I think it will be close.
There are very few modern motorcycle engines that are 400cc here in the states. There are 250's then we step right up to 500's.
The most common 400cc twin around here was the Honda Hawk. The operating speed of that engine is somewhere above 6000 rpm to get into any power. Peak power was somewhere above 9500. Below that, HP was considerably lower. If I remember right, Honda claimed 35HP or so from the Hawk. On the Honda engines that we dyno tested, their power claims were optimistic by about 30%. Operated well into the redline they got closer.
As an experiment, your idea has merit. It was unclear how you intended to operate the vehicle. I wouldn't want to rely on that particular configuration for a Daily Driver.
Until we get to very late model machines that are fuel injected, and utilize catalytic converters, motorcycles were much dirtier than cars. In grams per mile of particulates, a five litre v8 car was cleaner than a quarter litre four stroke street bike of just a few years ago. If raw fuel economy is the goal, you may be able to get pretty good results. As far as pollution, the civic engine is far better to the environment.
CJ