Do you mean 20 MPG city or highway? Many of our cars can do 20mpg highway and 13's, but I would think that very few hit that in the city. My car does around 16-17 city by my best guesstimate (and 13's with posi and traction, again, a educated guess). I have yet to add the lockup kit for the converter or swap in a 5 speed, so I will see if i can hit it in mine when money allows. The best city MPG I ever had in a car was 32 in a 5 speed 98 Sentra XE with a 115 hp, 1.6 liter 4. I don't know the highway on it as it only ever saw city, but the 95 Sentra I had with an auto and the same engine got 55mpg highway and 26 city. Both those cars were pretty slow and took 10-12 seconds to hit 60, but they weighed 2350lbs too.
The sad thing is that my Cutlass is actually about as good on gas in the city as my 4 cylinder truck now. The truck still runs fine, and has no codes that explain it, but the fuel economy has really gone in the sh*tter lately. I am thinking it finally is just getting old and there is not much I can do cheaply to bring it back to the 24mpg city it got when new. 330,000 miles of pizza delivery can't have done it any favors.
As for my combo, it's a basic 355 Chevy with Ok heads, 9.5:1 compression, a Quadrajet, performer intake, XE 262 cam, small tube headers, TH 200 4R trans, and a 3.23 gear. The car was planned with a set of parameters which included fuel economy goals, drivaeability, and running low 14's to high 13's. It was planned out as a pizza delivery car, so it could not be too radical. As for the performance, it makes around 350hp and 410-425 ft/lbs depending on if you go by the sim or the similar engine built by Comp Cams to demo this cam. The real treat is that it makes 400+ ft/lbs from 2,000 RPM until around 5,000 RPM, so it requires very little throttle to get it going. The Quadrajet also adds to the excellent low speed throttle response too as the tiny primaries let the idle be lower and give it a hair trigger throttle due to the strong vacuum signal. The lower idle speed also contributes to better fuel economy than a square flange carb can give. If I were to plan it again today, it would get a small LS engine like a 4.9 or 5.3 instead of the GEN I SBC, and I would run it off a Megasquirt ECU instead of a carburetor. Why go with the smaller engines? The LS is a very efficient design, and produces very good power to displacement ratio numbers. Since 350hp is all that is needed, a smaller engine is fine. Plus, as fuel economy is an equal part of the equation, a smaller engine naturally uses less fuel than a larger one of the same family, all else being equal. After all, what good is a fast car if you can't afford the gas to actually drive it? Also, aerodynamics do not play a huge role in low speed driving, otherwise I would have built something different.