Does anyone remember that the speedo was once accurate? It does still seem like you got a bad speedo. Who or whom did you get it from? A reputable vendor with a guarantee, or a guy with a short cigar at the swap meet?
People do say run them with a drill but you already know it registers. Nobody knows what rpm on the drill would be what MPH on the gauge so it's useless to check for accuracy. Are all of the mating gears in the tranny steel except the replacement? What is the part no./vendor for the replacement gear you bought?

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn