m 21 speedo gear while I'm at it

[Dennis D.]

Did the R&R on the extension bushing with the extension off. Want to try to correct the 10 mph difference in my speedo reading 70 when the actual is 60.

Didn't count the teeth, but its a green speedo gear with a green ring on the main shaft. I've tried changing only the small gear color with no success. Seem to remember reading in the archives about changing the ring gear as well. Any formula for this fix?

[Joe L.]

Re: m 21 speedo gear while I'm at it

Dennis-----

The green nylon DRIVE gear is the larger diameter gear which mates with 18 to 22 tooth (silver) DRIVEN gears.

What is the rear end ratio of the car and what size tires are you using? Also, what DRIVEN gear are you currently using?

[Scott Marzahl]

Joe, are there gears available for tire changes

Joe, if one was to install 60 series tires, with a 3:36 rear end, is there a speedo gear set to compensate for the difference in tire size?

[Dennis D.]

Re: m 21 speedo gear while I'm at it

Joe

235/60/15 with a 370 rear. Had a green gear,(21 tooth I believe). I've tried a blue, a red, and a black. Not sure of the tooth counts. NG

[Joe L.]

Re: Joe, are there gears available for tire change

Scott-----

Usually, there is a gear that will compensate among the available gears. Of course, without a made-to-order adapter it's usually not possible to get the speedometer accuracy greater than + or - 3%, or so. That's true with the factory original gear set-up, too. There are no "X-and-1/2 tooth gears".

[Joe L.]

Re: m 21 speedo gear while I'm at it

Dennis-----

The green DRIVEN gear is a 22 tooth gear. It is NOT compatible with your light green, GM #6261794 DRIVE gear. The black DRIVEN gear is a 23 tooth gear; it's not compatible with your 6261794 DRIVE gear, either. The blue 20 tooth and red 21 tooth DRIVEN gears are compatible with your DRIVE gear, but they won't give you a correct speedo calibration with youre tires and rear end ratio. The 6261794 DRIVE gear should have an OD of 1.85".

For your rear ratio and tire size you'll need to install a light blue DRIVE gear of GM #6261795. Unfortunately, this gear is GM-discontinued. However, I think that it's available from speedometer shops and otherwise in the aftermarket. This gear has an OD of 1.77". With the 6261795 you'll want to use either a 22 tooth green DRIVEN gear of GM #3860345 or a 23 tooth black DRIVEN gear of GM #3860346. One of these 2 should give you the closest calibration possible with stock gears. Use a portable GPS for calibration purposes.

[Duke W.]

Re: Joe, are there gears available for tire change

You need to find out the revs per mile for the installed tires whose size you did not mention.

Then you compute the percent difference from OE revs per mile, and all C2s are set up for the 6.70-15 760 revs per mile even though '65-up OE 7.75-15s were 775 revs per mile. Chances are your 60 series have greater revs/mi, so the speedo will read high and to slow it down you need a driven gear with MORE teeth that is compatible with the installed drive gear.

One tooth is about five percent, so you go with whatever is closest depending upon whether you want the speedo high or low.

The revs per mile for your tires should be listed at The Tire Rack or mfgs. web site. If the tire is out of production check other tires of the same size and pick an average or representative example.

Duke

[Dennis D.]

Re: m 21 speedo gear while I'm at it

Joe

The old eyes are going. I measured the drive gear, its 1.77 (the green and the blue look similar in color to me) Anyway, I had the green driven gear,(22 tooth), in there. Could me problem be with the speedometer?

BTW-- I meant to tell you its was to have finally put the face with the name when we met in Carlisle. Hope you enjoyed your family trip to the north east.

[Joe L.]

Re: m 21 speedo gear while I'm at it

Dennis-----

I doubt a speedometer instrument problem causing an error like this, but I suppose it's possible. The green gear or the black gear should give you a pretty accurate speedometer reading. Have you calibrated against GPS? You could also try a yellow 24 tooth gear of GM #3860347. You shouldn't need this gear, but you could try it. If your speedometer reading stays the same or doesn't come into range, then, perhaps, there is a problem with the instrument.