Speedo on a the mustangs uses a gear and a cable to turn the needle. If you swap gears, you will need the $7.95 from 5.0 resto or the like to correct for the change.
What year car? Which gears? I am assuming that you are posting in this section that you have a 5.0 and not a newer mustang. So all you nead to correce the speedo is a new gear like said above. The newer mustangs need reprograming but that would be a question for the 4.6 forum
Well in that case all you need to do is go to your local jeep dealer and get the 23 tooth gear. That is the one you will need to correct(or close enough) both 3.73 and 4.10 gears in your car. I am assuming it is a manual.
With a 93 5.0 and the 8 tooth drive gear, you won't ever have a correct speedo again. The 23 tooth is the highest driven gear you can get, which will only be correct for a 3.55 rear gear. That is if it's a T-5.
Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy with within 1%-3%. The drawback is that it isn't cheap.
Please excuse the typo (their's, not mine) in the following picture - it should read Custom ratio adapters, not Custom radio adapters.