I don't think it would help the mpg. I say this because I looked up the EPA fuel economy numbers for my car (turbo 5-cylinder 123) and the equivalent 240D, and mine was actually rated about 1 mpg higher in city and highway. The turbo motors apparently have enough of an efficiency gain that you can have the fifth cylinder without a fuel economy loss.

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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.