Speedo driven gears are sold in a range of colors which indicate the number of teeth. I'd assume that the gear would be specific to the transmission. Unless you can find someone to specifically advise you, a speedo repair shop should be consulted. Check the Yellow Pages in the phone book.
The T-5's ( and all the later model manual transmission Fords) use the same gears the Toploaders did.( both drive and driven gears) Ditto on automatics up to the electronic models) Figure the diameter of your tires, then multiply that times pi ( 3.1416) then divide into 63360 ( # of inches in a mile)This figure will give you the tire revs per mile. Then multipy the rear gear ratio X tire revs per mile X # of teeth on the drive gear ( this is the one on the output shaft) then divide by 1000 and get the gear with the closest # of teeth
To find the tire's diameter, multiply the width ( 245 in your case) times the aspect ratio ( 50 or .50) This tells you the side wall height. Then multiply times 2 ( you have two side walls , top and bottom) then divide by 25.4 to convert millimeters to inches. Then add that sum to the wheel diameter ( 16 inches)