[aerohead]

*With less than a foot added for a nose 'cleanup' a 'comprehensive' streamlining kit should net you around a 35% HWY-only mpg improvement.
*If your willing to put up with 3-feet of boat tail on top of that,you could see a 45% HWY-only mpg improvement.
*A little longer tail might net you a bit more.
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*Gearing will push your mileage higher.
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*I know nothing of what radials might be out there which might show up,mileage-wise.
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*Radials,O.D.transmission,and taller rear axle pushed my '62 Dodge D100 from 11mpg at 50 mph,to 16 mpg at 65 mph.
*A nose,full wheelcovers,,front gap-fillers,rear skirts,fan delete,and aero shell got it to 21 mpg at 65 mph,nearly doubling the HWY mpg.A belly pan would have pushed it over 2X.
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Rings are shot and I've probably got a burnt exhaust valve on one cylinder.She'd no doubt do better with a fresh engine.
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*The T-100 will do 36.5 mpg @ 65 mph with the 30" boat tail.
*31.8 mpg @ 75 mph.
*Originally,she was returning 23.3 mpg @ 70 mpg.
*No mods have been done to truck other than aero.