So I'm running the stock speedometer in my '54 with a 350 trans and a Nova rearend. I've always just put up with the fact that 60 mph is really 70, but now with more A-holes on the highway and with further commutes to car shows, I have to ask, is there a way to make my stock speedo read correctly? Thanks!
They make the plastic gears with a different number of teeth, but with ten off you might need a reducer.
http://www.tciauto.com/tc/transmission-accessories/speedometer-gears.html What I do is first calculate how far off the odometer is, by driving over a measured mile, or by using a GPS. If it's reading slow like yours is, that means you need fewer teeth on the driven gear. If it's off by 14%, then you need 14% fewer teeth. Next look at what gear is in it now, remove the cable from the transmission, remove the bolt and pull out the sleeve and gear. If it has 17 teeth, you'd multiply by .14 and that's the number of teeth you need to "lose". in this case 2.38, but you round it down to two, and you'd need a 15 tooth gear. If you get to a point where you can't get a gear with the right number of teeth, then you have to change the driving gear. that requires removing the driveshaft, and the tailhousing, and changing the gear that's on the output shaft. It's retained by a spring clip. TCI as well as Summit and Jegs sell the gears, but you might try a local transmission shop too.
If the right driven gear is not available we can build a speedometer ratio adapter that goes before the cable to correct the error, so you don't have to pull the tail shaft and change the inner drive gear.
The adapter box is a good way to fix it without getting ATF in your hair. Use a GPS to get an accurate reading of how far it's off by. Being a cheapskate parts collector, I swap out the gears. But I also have a few old adapter boxes I've rebuilt to work in my 55 when I swap out the rearend gears, so I just have to add the appropriate adapter to get the speedo to read correctly.
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