That car has a 4 cylinder engine that is the same as the 190E 2.3.

A 190E automatic can approach 30mpg on the highway.

Figure your car has more weight and frontal area, so your results will be less.

How much is a variable, but I'd start at 10% less.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles