"Speedometer reducing gearbox"--tell me all I need to know

[Mike E.]

I'm working on a 71 with a 4.11 rear (factory original), and I know on p. 239 of the present 70-72 JG it says "Cars with either 4.11 or 4.56 axle ratios have a reducing gearbox attached to the speedometer cable outlet on the transmission." 68-69 JG has same information. Previous owner said "the speedometer is off", and in the box of parts there is a right-angle adaptor that will fit the trans and the speedo cable housing. How do I know if that is correct? What gear color should the gear in the transmission be? What should the adaptor say on it? What ratio on the adaptor? Straight on or 90 Degree? I have never worked with one of these so I am a blank slate. Fill me in on what I need to know, so I do this only once, and do it correctly. I had a '62 with a factory 4.56, but didn't have anything like this.
Thanks!

[Joe L.]

Re: "Speedometer reducing gearbox"--tell me all I need to know

I'm working on a 71 with a 4.11 rear (factory original), and I know on p. 239 of the present 70-72 JG it says "Cars with either 4.11 or 4.56 axle ratios have a reducing gearbox attached to the speedometer cable outlet on the transmission." 68-69 JG has same information. Previous owner said "the speedometer is off", and in the box of parts there is a right-angle adaptor that will fit the trans and the speedo cable housing. How do I know if that is correct? What gear color should the gear in the transmission be? What should the adaptor say on it? What ratio on the adaptor? Straight on or 90 Degree? I have never worked with one of these so I am a blank slate. Fill me in on what I need to know, so I do this only once, and do it correctly. I had a '62 with a factory 4.56, but didn't have anything like this.
Thanks!

Mike------

Depending on the year, certain rear gear ratios, particularly very low or very high numerical, require a speedometer ADAPTER for correct speedometer readings. I suppose GM reasoned that it was not worth developing special speedometer drive/driven gears for the very few number of vehicles using these ratios, especially in the 63+ period. So, ADAPTERS were used to provide correct speedometer calibration. These ADAPTERS are simply small gearboxes that provide a specific underdrive or overdrive ratio.

For 1971 Corvettes with 4.11:1 gear ratio the correct speedometer DRIVE gear (on the transmission mainshaft) was GM #3978758. ALL 1971 Corvettes with 4 speed transmission used this same drive gear.

The correct DRIVEN gear was GM #3987921. This is a 21 tooth gear and is color-coded RED.

The original ADAPTER used with this driven gear was GM #6477965. This number is usually embossed on the adapter body. This is a 90 degree configuration and the ratio is 1: .833 and this is also embossed on the adapter body. This adapter was discontinued by GM in October, 1976 and replaced by GM #1565208. The latter was discontinued without supersession more than 10 years ago.

By the way, for 1971's with 4:56 rear gear ratio speedometer DRIVEN gear GM #3987920 was used. This is a 20 tooth gear color coded blue. This was used in conjunction with ADAPTER GM #6477964. This was also a 90 degree ADAPTER of ratio 1: .733.

[Duke W.]

Re: "Speedometer reducing gearbox"--tell me all I need to know

At some point in the late sixties or early seventies NHTSA established a FMVSS with speedometer accuracy requirements. IIRC they were allowed to read up to 2-3 percent high, but could NOT read low.

So to correct some tire/axle ratio combinations that were not compliant the OEMs used Stewart Warner adapters on the transmission that brought the error into the allowable range. Somewhere in my files I have a S-W data sheet that shows all the available ratios and they are numerous.

Another way to correct for speedometer error, often caused by changing tire size, is to recalibrate the speedometer head. I did this with my Cosworth Vega when I switched to 920 revs/mile 205/60VR-15 replacement tires from the OE 873 revs/mile BR70-15 OE tires.

I had a speedometer shop recalibrate a few percent based on calculations I made backed up with testing so the speedo was dead on at freeway cursing speeds. This was done by altering the magnetic flux of the head. This can also be done to a Corvette mechanical tach to improve their accuacy.

The CV has an electric tach, and I also had it recalibrated to be dead on at 7000 revs, which caused it to be about 100 revs low at 3000 and 200 low in the idle range.

Duke