Audi A6 comes with adaptive air suspension which offers multiple driving modes, Out of them, one is Lift Mode, which increases the ride height by 25 mm (1 inch) to 145 mm (5.7 inches). The Lift Mode can be selected up to a speed of 80 km/h (50 mph). Once the vehicle exceeds 100 km/h (62 mph), the ride height is automatically lowered to the level indicated by the mode that had been previously selected. Other than this if we talk about alternate options, then there are several number of ways by which you can increase ground clearance. You may choose bigger tires, a leveling kit, coil spring assisters, coilovers, etc. Also, we would suggest you visit the nearest authorized service center because this work needs experts to do everything in a proper manner. You can click on the following link to see the details of the nearest service center and selecting your city accordingly - Service centre.
The ARAI claimed mileage of Audi A6 is 15.26 kmpl. However, the actual mileage totally depends on several factors like driving conditions, road conditions, tyre pressure, etc. In order to improve the mileage, we would suggest you drive slower. Driving fast can reduce your fuel efficiency by up to 33% if you are traveling above 60 mph.