I'm not sure what the exact numbers are, but compared to the 4-cylander 2002 Honda Accord I had to the V6 2007 Sonata I have now, the Accord got rather substantially better gas mileage in city (frequent stop and go) driving but the Sonata is getting better gas mileage on the highway. I could care less about gas mileage right now.

That Accord was bumpy, horrible suspension and the Sonata is a smooth luxury-type ride. I'll gladly spend a little more on gas for that. No more smaller compact-type cars for me. After driving a 1991 Caddy Sedan DeVille for 8 years - yes, even the Accord was too small for my liking. I'm only 35 years old, but the bigger, the wider, and the fatter the 4-door sedan, the better I like it.

 

My 2007 Sonata v6 gets an average of 30 mpg. Granted, I have almost 30,000 miles on it. Mostly hwy. However, it went to the shop today for transmission problem. Would not accelerate in drive. Tow driver got it to accelerate but engine light stayed on.
Test drove an 08 and 09 limited edition with the leather interior. Oh my my! Almost traded in!