TRD 720 Speedometer drive. in Antique and Classic Mack Trucks General Discussion Posted April 21, 2017 9 hours ago, eddeere said: All of this has got me confused. The speedometer on my B 61 never worked. When I pulled the engine and trans the cable had an adapter on the end that had a square drive on it about .111". The trans needed on that one that was .125 ". I got one from Bobs Speedometer. Have not driven the truck yet so not sure of the fix. Hope I do not have to go thru all of the above. I've parted a few trucks that had the small square offset boxes that are ratio adapters for errors on the speedometer when you change something like the rear end ratio, Freightrain posted a picture of one. I've had several problems over the years with the speedometer and the drive off the tail shaft. The pencil gears and spiral drive I posted mostly came from donors I've had. Those reside in the tail shaft housing Freightrain also posted and did a really nice electronic conversion to. The issue is determining where your problem lies and how you want to fix it. There are many solutions (GPS, electronic) that can cure these issues.