C,mon guys,
You should know better than that. Mileage is purely a function of consumption and weight. The more load, the more work your card does and subsequently the more gas it draws out. Then the Consumption itself greatly varies based on driving habits- and I dont mean City/Hwy. I used to have a bimmer with those computer/ gas gauges that quickly taught me those laws of physics. When I drive around running late gunning the gas and braking, consumption shoots up, When I mellow out 60-70, the gauge barely moves!
I suggest keep your tranny set to Econ versus sport- I know its tempting sometimes, but that way you get the most fuel efficiency and just follow common sense rules of driving and you'll increase mileage significantly. Having a tuned car, tires inflated etc. etc. helps too.