The driven gear was something I corrected a over month after the swap was complete. I noticed it seemed like the speedometer was off. It felt like the speedometer was reading faster than I was actually traveling. I decided to get on the highway and do a test to see if I was correct. I drove for 10 miles according to the mileposts and my odometer read 11.4 miles. This means I would need to increase the number of teeth on the driven gear to compensate. The driven gear rides on the output shaft and turns the speedometer cable which in turn moves the gauge on the dash. The driven gear housing slips into a hole on the side of the tailshaft and is held in with a clip and bolt. The gear is held on to the end of the cable with a metal clip and is readily available from several vendors. I actually got mine at the local Ford dealer for $13.
I divided 11.4 miles by 10 miles and got 114% as the increase needed. My original gear had 18 teeth so 18 x 1.14 = 20.52. Since they don't make a 21 tooth gear, I decided to try a 20. You also have to specify which way the gear turns so it may be easiest just to bring the gear into the parts store with you. I drove another 10-mile test after installing the new gear and my odometer was now reading 10.2 miles. This means when my speedometer says I'm going 60 mph I am actually traveling 61.2 mph. Probably as close as I will ever get it.
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