I'm with Larry on this one, I don't think there will be a significant mileage change. The car still has the same amount of drag drving down the road at highway speeds and that's what's going to be eating up fuel once you have it moving. I'm sure it is much more pleasant to drive now that the engine isn't screaming. 
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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k
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