Just finished doing a little researching and I came up with this. With the new redesign that is rumored to make the next gen Mustang smaller this would also make it lighter. The current V6 Mustang weighs in at a little over 3400lbs, the current 4cyl fusion weights in at a little over 3200lbs. If they take the 2.5L 4cyl from the Fusion and direct inject it and toss it in the Mustang this alone would drop lets say 50-75lbs bringing the weight to a little under 3400lbs. Now drop in the smaller 7.5in rear end will drop another 50-75lbs bringing the weight to right around 3300lbs. Now you toss on a smaller set of brakes and drop another 15-20lbs, stick on a set of 16x6 wheels with a low rolling resistance tire. Now just doing these weight saving measures alone with the current(2011) Mustang will bring it to within 60lbs of the 2012 Fusion which gets 33mpg highway. Now since the next gen Mustang is rumored to be smaller you can drop another couple hundred pounds off of that and bring it to around 3100lbs. Now mate a 6spd manual with a 6spd auto, as an option, with both being geared for fuel efficiency, and the rear gear ratio at 3.08(maybe use a 2.73 as an SFE option). Now since the engine would now be positioned front to bad instead of transversely the engines intake and exhaust will be able to flow more efficiently producing more power without a jump in fuel consumption(this was proven with the 3.7L). This could bring the power of the engine to just shy of 200hp(estimate) and maybe breaching the 200hp mark with the use of direct injection(would also yield better gas mileage). Doing this and you would have a car that would be quicker than the 05-10 base model(4.0L) and yield significantly better fuel mileage(could reach as high as 35-36mpg I estimate). I believe this would be a great option for those who want the styling of a Mustang but don't want/need high horsepower and the high insurance rates that come with more powerful Mustangs.
From a theoretical standpoint this looks to be a very solid idea but I don't know exactly how well it would work out in the real world as there are many factors that can't be accounted for.
Black and White Crew
Crew Member #2