In my 540 my mileage improved by 3mph from my normal conservative street driving to "some" (when ever i would remember) hypermileing.
The key isn't driving like a granny. it's coasting. It's technically not allowed. But I would be going most places coasting in neutral. With the slight grade in my area, one of the directions I would coast at the speed limit for about 7 miles. at 800rpm, that gets you good mileage. also coasting once you reach the speed limit, and coasting from traffic light to light. the goal is to not have to use brakes. brakes use up momentum, and you'd only end up sitting there anyway. Also shutting the car down at large traffic lights.
point is, I'm back to driving 75 on the freeways, and having some fun in the turns/on-ramps