Speedometer gear drive ratios

**velocedoc** · #1 (**permalink**) 01-06-2007, 04:47 PM

I have encountered a wierd problem. My wifes spider from day one didn't have a speedometer. So I bought off of ebay a speedometer with about the same mileage. I checked it before I put it in and it worked just fine. After driving the car, the speedometer still didn't work. Ok, replace the cable. Nope, still dead speedometer. Hmmm, must be the drive unit. Removed the drive unit today. No noticable problem.

The problem is in the drive unit itself. The inside is stripped or rounded out so it won't engage the cable. I was going to remove the drive gear and swap out the center parts but I can't get the *R&#&$* pin out. It would be easier for me to swap the rear coupler and speedometer drive unit if I can find a match amongst the parts I have here.

I tried to insert the other drive units but they won't go into the rear housing and mesh with the existing rear coupler.

I tried to find amongst my odo drive units a match, but all that I have have physically bigger drive gears, as the one I removed was smaller in diameter and was stamped 9/18. There are two other drives...9/13 and 9/21.
Ok...so which fits a 76 spider?

bianchi1 · #2 (**permalink**) 01-06-2007, 07:38 PM

criss..i have the one i pulled out of my 84 box.. it is stamped...9/18 one way and 6/18 the other way. are these not counting the final drive? when i took mine apart the final drive to the speedo gear is on the thingl that holds the donut on..as you know my speedo is off some.. but my top speed now is 138 mph.

.. at home with a cold.. rats. and today got to be nice a warm.

**velocedoc** · #3 (**permalink**) 01-07-2007, 12:10 AM

This is probably one of those obscure questions that will take someone that is knowledgable about the differences in the gear ratios vs the out put drive units for the odometer. I know that a 4:10 rear axle has one drive ratio, a 4:55 has another drive ratio and a 5:12 has a different one. Sometimes people swap out rear axle ratios and forget to swap the odo drive and wind up with alot of speedometer error.
I suspect that I will just wind up dropping out the rear output yoke and dropping in a matching drive unit, so the wife will have a speedometer so she won't get pulled over by the CHP. Some speedometer is better than none.

Bill77 · #4 (**permalink**) 01-07-2007, 01:49 PM

My '77 Spider has a white plastic sleeve that slides into the hole in the trans case just before screwing in the cable end fitting. Could the white plastic sleeve be missing on your car?

**velocedoc** · #5 (**permalink**) 01-07-2007, 08:44 PM

Quote:

Originally Posted by Bill77

My '77 Spider has a white plastic sleeve that slides into the hole in the trans case just before screwing in the cable end fitting. Could the white plastic sleeve be missing on your car?

Alleggerita · #6 (**permalink**) 03-28-2007, 10:19 PM

Quote:

Originally Posted by velocedoc

I tried to find amongst my odo drive units a match, but all that I have have physically bigger drive gears, as the one I removed was smaller in diameter and was stamped 9/18. There are two other drives...9/13 and 9/21.
Ok...so which fits a 76 spider?

**velocedoc** · #7 (**permalink**) 03-28-2007, 10:47 PM

Good information. Thank you for making the effort to sort this out. I did the math and a 9/41= 4.55 8/41= 5.12 10/41= 4.10 9/18 - 2.0 I have no clue. I will take a picture tomorrow of the drive gear with the stamp on it, maybe someone will know what it goes to.

Alex · #8 (**permalink**) 03-29-2007, 04:05 AM

I would also like to know about the 10/43 speedo drive gear.

Alex.

**alfaparticle** · #9 (**permalink**) 03-29-2007, 06:28 AM

Alleggerita:
A little light turned on in my head when I read your post. The speedometer in my 79 spider is overly optimistic. The gearbox in it is one that came out of a 1970, Euro 1750 spider and I am pretty sure that it had a 4.3 axle. I probably have the speedometer drive that you want in that gearbox. I have plans to swap out the axle and gearbox to a 4.1 from an 82 Spider, but I am not sure when that will happen.
Ed Prytherch
74 GTV
79 Spider
82 Spider parts car
88 milano Verde

vsharp · #10 (**permalink**) 03-29-2007, 06:39 AM

Both the speedo cable-drive pinion AND the helical gear that drives it vary in tooth number to achieve the corrections for the various diff ratios.
As far as I have been able to work out, for 14" wheels:
10/41 (4.1 ratio) 13/6, cable pinion and helical respectivley.
10/43 (4.3 " " ) 18/8 " " " " " "
9/41 (4.56 " ) 21/9 " " " " " "
A bit more input from others is needed to complete the other various ratios.
NOTE: If you put the wrong pinion and helical together, they won't mesh correctly and a stripped cable-drive pinion gear is the usual result!!!! BE WARNED, I have a box full of shagged ones!
Vince.

Alleggerita · #11 (**permalink**) 03-29-2007, 09:23 AM

Quote:

Originally Posted by vsharp

Both the speedo cable-drive pinion AND the helical gear that drives it vary in tooth number to achieve the corrections for the various diff ratios.
As far as I have been able to work out, for 14" wheels:
10/41 (4.1 ratio) 13/6, cable pinion and helical respectivley.
10/43 (4.3 " " ) 18/8 " " " " " "
9/41 (4.56 " ) 21/9 " " " " " "
A bit more input from others is needed to complete the other various ratios.
NOTE: If you put the wrong pinion and helical together, they won't mesh correctly and a stripped cable-drive pinion gear is the usual result!!!! BE WARNED, I have a box full of shagged ones!
Vince.

**velocedoc** · #12 (**permalink**) 03-29-2007, 02:07 PM

Alleggerita, I have what you are looking for.

Wanna trade? I need a 21/9. Go here... http://www.veloceregister.net/speedodrive.jpg

Alleggerita · #13 (**permalink**) 03-29-2007, 09:25 PM

Hi Christopher,

I kind of offered the 9/21 to Ed already, though he let me know that he really would prefer the 13/6. Anyway, the 9/21 is sitting in the transmission right now, so, I'll have to pull it anyway. Also I may be able to come up with another.

Anyway, I do have 2 helicals (6 teeth), one attached to the transmission output fork, one alone, for a 10/41 but so far am missing the pinions.

I actually have the helical for the 10/43 but am missing the pinion (8 teeth).

Also I have the helical for a 9/43 which has 9 teeth - I don't know what the pinion is supposed to be.

Anyway, ideally I would like to swap the 10/41 pieces (6 tooth helicals) for a 9/43 (? teeth) and 10/43 pinion (8 tooth). My 9/41 (21/9) set is also up for grabs potentially if Ed doesn't want it but I would ideally like to swap it complete with output fork and speedodrive unit so I don't have to take it apart.

Finally, I got a 8/41 (21/8) set to swap, either with speedodrive and output fork or without - the one with output fork is in a car with the trans - I'll let go of only one of them.