Not to one up anyone, but I recently returned from my annual 2300 mile round trip from ATL to NY. Averaged 25MPG. I'm a very conserative driver.
I have a non S/C car.
This includes a lot of cruise control, a full trunk with a loaded large cooler, and it was cool the week I was on the road. Not much stop and go traffic, but did drive around town on LI and upstate. This car has always been great on gas. I usually average 19 around town in ATL traffic.
My mods also help:
Stock gears
Ford shorty headers
X pipe
Flowmasters
Under drive pully's