Replacing the Speedometer’s Driven Gear

In an earlier post a while back I mentioned that the previous owner had replaced the 170 and 2 speed automatic with a 200 and C4 (three speed automatic).  The drive gear in the C4 differed from the previous merc-o-matic, so the speedometer was off–way off.  I estimated that we needed a 21 tooth or maybe even 23 tooth driven gear.  The driven gear is the gear at the end of the speedometer cable and turns to move the speedometer’s needle.  Replacing it on a C4 is very simple.  First, we ordered new gears (21 and 23 tooth gears), a replacement o ring, and a replacement clip:


So, out to the driveway we went.  All it takes is tracing the cable to the driver’s side of the transmission.  There will be a bracket with one bolt to loosen.  Loosen it and slide the bracket either off or down the cable (depending on the style of bracket).  Then, pull the cable out.  In my case, I had to use a screwdriver to help pry it loose a bit.  Once you do that, you should see something like this:


Use a needle nose pliers to slip off the clip holding the gear to the cable.  Pay attention to how the clip is on.  You want to reinstall it the same or, if it’s broken, replace it with a new one facing the same way.  Simply press on the new driven gear, slide the clip on, push the gear back into the transmission, and rebolt the bracket.  That’s literally all there is to this job.

In our case, I tried the 21 tooth gear first and found I was still off by about 10%, give or take (going 27 when the radar said 30).  I put in the 23 tooth and it was spot on.  That’s a good thing, because that’s as high up as it goes.  I would’ve needed a reducer after that.


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