Kev, I honestly wouldn`t swap that rear out. The trans shift points, ECM, speedo are calibrated for that rear. That Van is heavy, Lowering to 3:55`s will hurt performance and mileage unless you relfash the ecu.
I have the 4.6 with the 3:73 rear in a cargo van and just upgraded to aftermarket 18" wheels with a slightly taller tire and i can definetly notice the difference.
You should be getting anywhere between 13-16 MPG. Even when trying to economize for fuel consumption I doubt it would do better. Remember the van is a flying brick...
no matter what I do my average is between 13.5-15.5
If I`m on a long cruise I can reach 16-17.
But when towing it drops...
I think that 4:11 is exactly what that van needs. it`s actually quicker acceleration which will get you up to speed then kick back and cruise is your best bet for good MPG.
If you want to fine tune, and the engine is in very good running/mechanical condition, then I would think of purchasing a flash tuner and making some adjustments. Good Luck!...
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2007 E-250 Super Duty Van
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