I can't answer your question but I once saw a chart somewhere on the net that listed all the different speedo gears but I can't find it now. I've been looking for it for a similar problem. Maybe someone has it bookmarked and can share a link.
'64 swb stepside (gone) '57 1 ton '53 phone truck '59 swb '46 1 1/2 ton '68 swb gmc
