first off i think the cars are out there, but the marketing hasnt caught up.
we really need to take out all the crap that comes in cars. reduce the weight.
then we can use the smart 900cc engine and couple that with a supercharger for extra power, and use it to power something like a honda civic. you really only need power for acceleration and climbing. cars use very little power when actually cruising, certainly at legal speeds. when you start getting into the realm of very high speeds is where you need real horsepower.
so imagine a (light) hybrid honda civic/ ford focus/ powered by electric motors at low speed, say up to 20mph. the IC engine kicks in at higher speeds, just like a hybrid. in acceleration mode it has a supercharger. in light cruising mode it runs the atkinson cycle, has stop/ start when cruising, and elecctrically disengages the supercharger when not needed.
with proper gearing, an (EK weight) hybrid would easily do 50mpg combined.
in fact, i cant understand why someone hasnt already taken a 93 civic DX coupe ( not quite the lightest (HX hatchback) but better aerodynamics) and done something similar.
use a supercharged smart engine. 185/ 70-14 tires. you may not even need to change the gearing at all as the smart has much smaller tires than the honda civic. the bigger honda civic tires would essentially multiply the gearing for freeway cruising. use golf cart motors (as suggested) for the rear wheels, place some capacitors and odyssey batteries under the back seat. clean up the aero a little (with a front air dam, side skirts, underbelly enhancing, hidden wipers (raised trailing edge on hood.) smaller side mirrors etc) and youd end up with a car that should easily beat the fuel consumption of todays cars (which are beaten by yesterdays cars) and still be remotely desireable.
then you have the normal series of engines. for more excitement/ conventionality as it were.
moreover, if honda does a ford transit connect with thier stream, you have multiple variants which could be powered by the same drivetrains.
I learned an interesting fact about CAT engines: the crank journals are divided, split and offset slightly. They do this for their V8s. Wonder what that does.
i think the offset crank journals reduce piston slap. obviously there is only pressure on the pistons on one downstroke, the compression stroke. because of thermal expansion there has to be a slight gap between the piston and the cylinder walls. but that introduces piston slap. offset the crank slightly towards the rotation/ pressure side and you reduce piston slap.
its more noticable when the engine is cold.