I have an 02 six speed with 3.73 rear and just x-springs. I get 19-19.5 at 70 mph on the highway. That is with driving very cautiously for mpg
. I also have a cap with a slight bump. So I would think the estimates of 15-16 are probably accurate.
2002 f-350 PSD, 4x4, cc short bed, 6spd, ccv mod, airdog, AIS filter, isspro ev and scangauges, detroit TT, ITP boost annihalator, ITP RR, bellowed up-pipes, ats housing, turbomaster, 3.5 dp magnaflow, very happy with ultra smooth and linear PHP 50t, 80DD and "whisper" tunes.