76 D100 / 318 / A833-OD / 8 1/4"-3:55 / 275/60/15 (28" dia) on 9" wheels. I used the chart on Brewer's website to determine the speedometer gear I needed as 33 tooth. Brewer's Speedometer gears

I picked this little jewel up in February out of Oregon. Due to transmission gross leaking, title issues, work, & life, I have just got it on the road. Transmission rebuilt and sealed thanks to parts and tech expertise from Brewer's.

Speedometer shows about 9 mph too fast at 60 mph or about 69 mph. Checked against two different phone speed apps. I also jacked the rear up, re-checked the tag on the diff and did a manual check. Yep, 3 1/2 turns of the drive shaft equals one turn of the tires.

Bought and installed a tach yesterday. Sure enough the rpms match what everything else indicates it should. About 1850 rpm at 60 mph. OD is .76 for those that don't know it off the top of your head.

Speedometer was replaced with one from a salvage yard that I lucked upon while there getting parts for my 99 2500. The one in it was sticking horribly around 50 mph and when I pulled it to attempt some lubrication, the needle was so brittle in wound up in 7 pieces.

When I calculate the number of teeth I need per the percent error or the longer set of calcs to get to speedometer cable rpm it indicates that instead of the standard 1001 cable rpm for 60 mph, I need 875 cable rpm or a 38 tooth gear. Chart indicates this is a match for 4:10 gears.

What am I missing? Surely / hopefully something simple I am just overlooking. TIA.