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Vintage Ford Mustang Tech Advice - July 2014
Beyond the Basics
Speedo CorrectionI have a '67 fastback with a C4 transmission from a '71 Mustang. I also have a Traction-Lok rear with 3.50 gears. The problem is the speedometer. All of my research indicates the way to calibrate the speedo is to change the gear at the transmission. The drive gear in the transmission is machined into the output shaft and can't be changed easily. The plastic driven gear on the end of the speedometer cable is available in different teeth counts to adjust for different gear ratios and tire sizes. My calculations state I need a 21-tooth driven gear to correct my speedometer. My research shows that the 21-tooth gears are thin and that the gear will not last long. Should I try the 21-tooth gear or is there another way to correct my speedometer?
Oklahoma City, OK
Factory driven gears were available from 16 through 21 teeth and can differ between manual and automatic transmission applications. Three-speed manual and automatic transmissions have the speedometer cable entering the driver's side of the transmission case. Four-speed manual transmissions have the speedometer cable entering the passenger side of the transmission. The angle of the gear teeth on the driven gear differs accordingly. The driven gears with the teeth angled toward the left (sitting upright on a table) are for four-speed applications, while driven gears with the teeth angled towards the right are for the three-speed manual and automatics. If the incorrect gear is installed it will be ruined in short order. This is an easy mistake to make and is perhaps creating the issue of early failures you are reading about. Install a 21-tooth gear angled towards the right and your speedometer should be just fine.
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