For 63 in a 250 you probably have 4:56's for gears. Both of my 61 F250's both have that ratio. They'll run over 60mph no problem but their screaming pretty good. No lugging at that point!! Surprisingly my one if driven 60-63mph will pull down 13-15 mpg with turning 2800-3000 rpm's. If I were ever to get it changed down to the 3:73's like what your doing it should do 16mpg I would think.

                                 

Michael

Sandy Valley, NV