I find two things that really vary my mileage. Speed and altitude. The faster I go the more it goes down. The higher I go, the more the mileage goes up. If you keep your speed down to 55-60 and keep your acceleration to a moderate, you should easily be able to get over 70 (US) mpg. If you want to cruise 60-65 on the freeway, expect the mileage to drop to about 68. If you what to do 70, look for it to drop to near 60. If you can spend a lot of time above 4000 feet or so, you can look for mid to high 70s.
Or so my experience goes on a 2011 G-5.7200 miles.
Bare