What type of gear was used in a 1990 f250 3/4 ton with a 4.9 engine?

What transmission do you have? If you a direct drive transmission (i.e. non-overdrive automatic or 4 speed manual), it sounds like you have 4.56's. You would be at 3112 rpm's with stock tires. 3.55's or 3.73 will put you the rpm range you want. With 3.55's your rpm's will be at 2423, with 3.73 you will be at 2546 rpm's. Both gears should be alright, but if you do any towing or hauling, the 3.73's would do ya a little better. If you have an overdrive transmission, your down in the 5's. Either way those gears probably not your stock gears.