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Speedometer Accuracy*
by Bob Adler

Accurate speedometer gears are a real weak link in any transmission swap. Compounding the problem are the non-interchange ability of three speed and four speed components, and a specific gear set for each axle ratio and tire size combination. Note tire sizes are listed as factory regular production options (RPO) and not dealer installed accessories. This is because there is a likelihood that the transmission will need disassembly to change speedometer drive gear; an operation best accomplished during initial assembly. All is not hopeless; there are some tricks available for tackling this problem. This article deals specifically with Advance Design trucks, 1948 to 54, but should be useful to anyone who desires a more precise speedometer.

The speedometer drive gear is mounted on the transmission output shaft. Chevrolet calls this the main drive gear. On torque tube three speeds, both car and truck, it slides out after the universal joint is split, the front half is removed, and the trunnion support bushing is pressed out. While there is no substitute for consulting the shop manual, keep in mind they describe total transmission disassembly with specialized presses. Frequently partial disassembly is better and a brass hammer is substituted for the specialized presses. On four speed transmissions the speedo drive gear is pressed onto the universal joint yoke. The four speed drive gear is physically larger than the 3 speed drive gear. Some have a clockwise helix, others counterclockwise. If the wrong helix is substituted, the gears work for a while until the teeth strip out, then speedometer action becomes erratic. Both three and four speed trucks from 1948 to 1954 mount the driven gear above the drive gear centerline, and mount from the driver's side of the truck. Earlier four speeds are totally different and are not likely to be confused but later three speeds started mounting driven gears from the passenger side, or below the centerline on the driver's side.

This meant the helix on the driven gear was inverted. There are a few acceptable driven gear ratios for a single given drive gear, so occasionally getting an accurate speedometer is as easy as swapping driven gears with no major disassembly required. The four speed transmissions usually came out of larger trucks. When transplanted into half tons the ratio is off by a large percentage. Part 3692232 is the gear combination that fits 1948 to 53 half tons with 4.11 rear end and 6.00-16 tires, using three speed transmission. The drive gear has four teeth and driven gear has twelve teeth. For the four speed trans part 3689223 is used, and also has four and twelve teeth. This unit is also used on the three quarter ton truck with 7.00-17 tires and 4.56 rear axle. the one ton truck with 7.50-17 tires and 5.14 rear also used this part number with an external adapter to slightly change the ratio. If the existing gears are off by a known percentage, it's possible to calculate how many driven teeth are needed for the existing drive gear. Hunting for that driven gear would bring the speedometer closer to accurate. If there is more than a couple of teeth difference between existing and desired driven gear, expect the teeth to be incompatible and grind each other up.

Another possibility is purchasing a speedometer adapter from a Chevrolet dealer. These are currently available, and frequently still used on trucks. Calculate the percent of inaccuracy, and ask for a speedometer adapter to compensate. These are found in parts group 4.329. There are many input to output ratios available, both to speed up and slow down the speedometer cable. Some trucks, as noted above, came with these installed directly from the factory. They screw in between the transmission and the speedometer cable. Some need a stubby shaft with key to transmit torque from driven gear into adapter. These are also available at the dealer. This should be considered a viable solution to maintain speedo accuracy, and is also easy to install, as no transmission disassembly is required. If the proper ratio is not available, it would be possible to mount two adapters in series if there is enough room for them to physically fit, although I do not believe this was an authorized factory tactic. This solution is easily reversed if it is unsatisfactory, or the proper gears are found later. If tire size is changed this is a good way to bring the speedo back to accuracy.

* Originally published in the Vintage Chevrolet Club of America, Inc.'s "Generator & Distributor", Aug. 1993, v32, no. 8, p28.

Bob Adler is owner of Adler's Antique
Autos, Stephentown, New York, and
specializes in GM truck restoration.
He can be reached at 518-733-5749.

Adler's Antique Autos