Probably right about the economy. I dont doubt he could make the
concept work, however.

FIL has a Sonoma with the small 4 cylinder engine, and it gets less than
20 mpg.  Our 3800 V6 26 mpg or better than that...BUT, the V6 cruises at
70 mph at about 1800-2000 rpm.

The I4 spins up to as much as 3500 rpm to maintain 55 mph on a steep
hill, and runs 2500 or better to do 70 on the flat and level.

Overall gear ratio, aerodynamics, engine management, and perhaps
other factors apparently have to work well together to get good mileage
and reasonable power.

Thanks for the input guys.

I'm not worried about an emissions test, my state does not have one.
(we can also own GUNS w/o a license!!)

Buying something defeats the purpose, to save money on gas. If I bought
a four thousand dollar vehicle to save .40cents/gallon, well do the
math. Payoff would be way down the road. (about 500 trips before I
started saving)

I'm already using a Honda 750 shadow that gets 50mpg, but come winter,
its damn cold.
It was not purchased for strickly saving gas, so the 3500 I paid for it
is not the same. (its one of my hobbies for 30 years)

A chevy lumina I had with the V6 aluminum engine regularly got 23-25mpg
to work, (30mpg on long trips.)

I don't believe there are any 2cly vehicles available in the states
that I know of. I would love to get a SMART car and tinker with it.
But, not available.

Of note, I was also thinking of putting a double flywheel (increase
weight somehow) If these little 4 clys were long stroke, I know it
would work. But the short stroke will really hurt the torque.

I respectfully disagree that the fuel milage would stay the same. If I
inject 50% less fuel, and expect lower power output, and decrease
speed, well below what it was designed for, I should use less fuel.
(Plus less weight) I do understand your thinking though. X-work must
have Y-fuel to perform.

I am considering using a motorcycle engine (water cooled) also. But the
coupling to the transmission input shaft would require a pillar block
bearing of some sort, and alignment would be critical.

I was just hoping someone else had tried something like this and I
could learn something. I don't know how the computer would react to
lower(?) discharge gas temps. What about the ignition pack? Would not
allowing two plugs to fire cause a default failure code or kill the ign
pack while trying to fire two non grounded plugs? Mass Airflow Sensor?
Hmmm,...idle valve?

Motorcycle engine is sounding better and better.