How bad are we talking? A W140 S500 is a big heavy car with a big thirsty engine. I would be surprised to see one running perfectly get much better than 20 mpg. Fueleconomy.gov predicts 14 city and 19 highway. An economy car it isn't.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.