In the EPA cycle, the smaller engines do better almost universally, in real world driving this is not necessarily the case. Real world driving requires a bit more performance sometimes than the EPA cycle (such as the on ramp to a freeway, passing, etc.) a 3.5L V6 may only need 35% throttle to get to speed in time while the 2.5L I4 is chugging along at 70%. This is also highly dependent on driving habits.
I hammer the gas 100% pretty much all the time in my Corolla and I still beat the EPA numbers :biggrin:
edit: until I reach the speed limit. I haven't had a speeding ticket in 4 years.
IIRC, small and big engines cross over at very high speeds due to gearing, but that doesn't apply to most modern cars because the slowest and shittiest car of today is still a lot better than midrange cars of yesteryear. Back in 1960 or whatever, a little 3 speed car with 60HP would be damn near redlining on the highway just to maintain speed, and yes they did get really bad mileage because of it. Old people are correct.... 50 years ago. Right now, the shittiest car you can buy has over 100HP. The only example I can find where a smaller car has worse mileage is the Honda Fit vs the Honda Civic. EPA's ratings for 2011 say the Fit does 35mpg on the highway while the Civic does 36 even though the Civic is heavier and has a lot more power.
When you're talking about a 170HP 4 cylinder vs a 250HP 6 cylinder, none of the above applies. Both have more than enough power to keep the car moving at 80mph at a low RPM without breaking a sweat.