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The numbers have yet to be published I think, but I don't believe it will be the same. Give the higher output of the JCW, I wouldn't be surprised if the mpg ratings are lower.
2007 MCS: 29 MPG city. 2014 JCW:24-25 MPG city (no AC). I'd say 5 MPG worse. Not as different on highway probably because you don't need the turbo boost. My tank is 14.2 gallons, I get about 300 miles city before the warning lights come on with this car.
I bet if I'd driven the MCS the way I drive the JCW I would have gotten 27 instead of 29, so the difference is probably more like 2-3mpg. The R56 was 1.8L and the F56 is 2.0 but it supposedly gets better mileage now.