Mine is also too optimistic on the MPG estimate. I have found it to be as high as 3.2 mpg higher than what the simple division of miles / gallons indicates, most of the times 2.1 mpg higher.
A big factor also in the fuel economy is
the tires. I changed tires fairly recently and I saw my fuel economy plummet just because of the new tires. I had the original Dunlop SP Sport 7000 V rated 215-60/R16 and replaced them with Cooper Zeon RS3-A 225-55/R16. Just in the fist tank I noticed a decrease of aprox 10-12%. Given the additional tire width, I would have expected 5-7%, but the Zeons gave a big hit.
Granted, the RS3-A are a better, probably much better performance tire, incredible in the dry, amazing in the rain, worse in the snow (expected), quieter, but the hit in the wallet is felt nonetheless.
Now, with that out of the way:
SP sport 7000 215, summer: 29.4 mpg avg combined, was able to pull 33 mpg on highway driving.
SP sport 7000 215, winter: 27.2 mpg avg combined, close to 30mpg on mostly highway.
RS3-A 225, summer: 27.3 mpg avg combined, was able to pull 30 mpg one time pure highway driving, 70mph, driving like a grandpa.
RS3-A 225, winter: 25.4 mpg avg combined... ouch!
Gas is usually regular 87 octane unleaded in SE Michigan with higher alcohol content. For driving habits, I have an extremely hard time driving the car easy, I always take the ramps and the twists as if I was racing
.