I get a little better than that highway, i.e., 19.5 - 22.5 highway, but around 17.5-19 city, on my 300 SEL which is bigger than your car. All depends on your driving habits.

Believe it or not my A/C does not have that much affect on my mileage, don't know why.

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'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)