[edit] I agree, by the way, that the cars are incredibly fun to drive, for being "fuel efficient."
I'd check under the hood just to make sure :) then fill up the tires, reset the average fuel economy, and baby it on the highway to see if you can get around the expected 40 mpg.
On a 50%-50% split, I would be in the mid 30s, usually 36MPG or 37MPG.
When I was 80% city, I would be around 32MPG on average.
That car does not have much low-end torque, so if you accelerate aggressively from stops you can definitely tank the MPG. I didn't see much impact from aggressive acceleration at highway speeds (e.g. going from 30MPH to 70MPH on an on-ramp, or pulling around a slow group of cars going from 45MPH-70+MPH). I did see a lot of impact from flooring it 0-30.
Hope this context is helpful.
Every time you touch your brakes you're throwing away the energy it took to get to that speed. Coasting through off ramps at highway speed is good for the environment.