I just got back from a 1,400 mile round trip from Louisville, Ky to the Pocano's. Averaged 33 MPG at 78 MPH/AC on and through the mountains with cruise control. On the way back, I didn't use cruise, just tried to keep it around 80 MPH and my mileage dropped to just over 30 MPG. Cruise gives you about 10% better mileage.

 

30 vs. 33 mpg is not so far apart that it can't be caused by other things. You'd think if you were just as smooth with the pedal as your cruise-control is, then it would be the same.

Part of that could be 80 vs 78 mph. Part might be different temperature that day, maybe different wind?

 

^You're right, it is close and I obviously cannot be as "smooth" applying the gas as cruise control can. I'm not sure just how much wind I passed in either direction. JK

 

I own a 1999 Honda Accord 6 cylinder with 164K miles and got 37.89 mpg on the 6 hour drive up from Indianapolis to Appleton, WI and 38.1 mpg on the return trip today. Rode on cruise most of the way. I was amazed at the numbers. I have replaced the original transmission and catalytic converter otherwise original equipment.