[ProDigit]

If I had an opportunity again, I'd love to try modding the Sym Wolf150.
A very nice, simple, and mod friendly bike.

Most 150cc or below bikes are chinese bikes, with their inherited problems of screws falling off while riding, chrome rusting, 3-5k miles before a valve adjustment (2k miles on the 50-100ccs), low engine life expectancy, bad electrical cable wire harness, etc...

The Sym Wolf 150 is Korean, not inheriting all those chinese flaws, and probably the perfect city commuter.
It can go on highways, but not too far. Forget interstates on this. It would be dangerous, unless going on the right lane tailgating trucks to reach 65-70mph.
It's a great tradeoff between power and mpg, hitting just right in the sweet spot for city rides.
eco modders would love doing sprocket changes to rev that 150 in the sweet 3k rpm range, while doing a nice 30-35mph, clutch allowing you might be able to get it to do 3k rpm @40mph, surpassing 110mpg when cruising comfy.

The Honda Rebel 250 slightly has too much power in the city, while just not enough for the interstates (7.5k rpm @ 85mph when ducked forward, or 6k rpm @75 mph is about the best you can get out of it, though it generally only can get 65-75mph reliably top speed, accounting for head wind and hills).
It's a great Highway bike, and with the correct gearing great for going 40-55mph all day long, and mpgs surpassing 85mpg. A great number, but not great enough, when 100+MPG is achievable!
It's engine is a lot better than the Suzuki GS500 in terms of low end torque, and low RPM performance.

The chinese bikes stock have 80+mpg, and can be modded for 100+mpg, but they have no oil filter, and their oil consumption is quite high.
from both an environmental and economical point of view, they aren't very good investments at all in the long run.
The increase in oil changes, offset all gains done in fuel MPGs.
The increase in valve adjustments, and repairs on broken parts will catch up on you in the long run. Their lower resale value makes them worth nickels at the end of their lifetime, which often is only 1/4th that of a Japanese bike.

For that reason, without trying it before, I'd probably nominate the Sym Wolf 150 as the perfect econo-bike for the city.

It'll be perfectly possible to mod any 125 to 150cc to do over 150mpg, but when gearing becomes too tall, it no longer becomes a practical bike to ride in city traffic.
Too tall gears cause bad acceleration, and difficulty departing from a dead stop.
Wind drag on small bikes increases exponentially from 35mph onward, so your sweet spot for max mpg would be 35mph on a 100cc, 40mph on a 150cc, 45mph on a 250cc (trying to keep revs between 2.5 and 3k rpm.