when using cruise control, it doesn't maintain speed

I know that it is normal to have fluctuations in speed due to hills and such, but this vehicle is under the set speed almost constantly and I think it is effecting the gas mileage. I only got an average of 14 mpg on my first 1/2 tank of gas (that's all the dealership would give me). I do mostly city driving, but the last 60 miles was highway. That is ridiculous for a 4 cylinder engine!