Fuel Economy?

I keep very close track of how much fuel my cars use. Needless to say the Mercedes uses more than the Miata. It hungers for fuel, craves it, thinks about it all the time, and not the cheap stuff, it wants the goods.Premium fuel is the only gas the Mercedes will accept, and it accepts a lot of it.
The Mercs gas tank is 70 litres which includes the reserve, the Miatas is half that and it takes the cheaper gas and it burns less how much less? Here's how it breaks down:


City Mileage:
Miata 9.8l/100km (24mpg)
Mercedes 14.1l/100km (17mpg)
Highway Mileage:
Miata 7.4l/100km (32mpg)
Mercedes 9.5l/100km (25mpg)
Combined Average:
Miata 8.7l/100km (27mpg)
Mercedes 13.7l/100km (17mpg)
Best Tank:
Miata 6.3l/100km (37mpg)
Mercedes 9.3l/100km (25mpg)
Worst Tank:
Miata 11.4l/100km (21mpg)
Mercedes 16.8l/100km (14mpg)

In six months of Miata ownership I spent $1162 on gas and travelled 11100km, that's 10 cents per kilometre. In six months of Mercedes ownership, I spent $1210 and drove 8693km, that's 14 cents per kilometer, 40% more. But the Merc is way more car than the Miata so it kind of works out. These numbers are also scewed because the Miata is used in the summer, and is driven for lot longer distances; average km per day was 531 (this number is heavily scewed by the 24hr mad drive to Vegas). The Mercedes averaged 47km daily. But gas has been much cheaper than last summer so if gas were to match last summer's pricing the Merc would be even more expensive to run. Still it's a Mercedes, it's not a toy and it doesn't fuck around.