Wrong converters, gears, shift points, aerodynamics, alignments (ever try pushing a car around the shop with a bad alignment then again after getting it fixed?) and much more can affect mph. How many times have we seen guys pick up big MPH after finding a fuel delivery problem? The only thing MPH VS weight calculators can tell you is how much HP (roughly) was put to use. It will not tell you how much your engine makes, more of a minimum. If your engine dyno was corrected properly and made 800 hp but the calculator says 400 hp that means you have some major CAR issues needing fixed. Would a 3000 lb studabaker hawk run the same MPH as a 3000 lb D-100 with the same drivetrain? They are both useful tools but should never be taken as gospel.