Speedo Drive Swap

[NCArachnid]

Does anyone know if you can change the speedo drive gear without dropping the transmission? Has anyone done this with the tranny in the car? Is there a trick?

Also, is it correct that a speedo drive gear from a mid '78 and earlier should be the same as the broken one in my '72? The speedometer should read with the same accuracy?

[BEEK]

the speedometer gear and the corresponding drive gear must be swapped as a set. ive never tryed this in a car. the speedometer gear must correspond with the rear end racio

[majicwrench]

Well, I have never done this is a Fiat that I can recall, but in a lot of cars, the drive gear and the driven gear do not need to be changed as a set. I have changed probably a hundred driven gears over the years, mostly in GM vehicles. Tire size, rear end ratio, the speedo gears, it all makes a difference, and sometimes you have to mix/match to come up with a accurate speedo.
Keith

[BEEK]

in a fiat 124 the gears must be changed as a set

[NCArachnid]

So to clarify, my 72 has a 4.30:1 rear gear. I need a speedo drive gear from another car that had a 4.30:1 rear gear. Right? Anyone changed one without pulling the trans?

[kmead]

I believe the gear that needs to be changed is the one inside the transmission. So no, you cannot replace the drive gear without removing the transmission.

[Ernie]

Jon,

Just to make sure both speedo drives are from cars with 4.30 gears -- there should be a red dot painted on the speedometer drive housing.

I think you can change the drive with the transmission in the car. It's going to be tight, but no harm trying. You could possibly loosen the transmission mount to get a bit more wiggle room.

Ernie

[kiznarsh]

Any updates to this?

Mine isn't working and the previous owner replaced the gauge and the cable, but still no dice. So it's gotta be the speedo drive itself and I managed to loosen it and attempted to pull it out, but there's hardly any room down there. The shaft that's inside the rear housing is close to 2" and there's only ~0.75" of clearance so...this guy still has no idea how fast he's going.

[124JOE]

i have a garmin it helped me figure out how far im off on speed

[kiznarsh]

I'm actually more interested in keeping track of mileage for maintenance purposes.

[vandor]

> So it's gotta be the speedo drive itself

To be sure you can disconnect the speedo cable, turn the driveshaft, and see if the output of the speedo drive is turning.

>and I managed to loosen it and attempted to pull it out, but there's hardly any room down there.

Working from memory I'd say there is no way to do this in the car. Likely not even if you unbolt the tranny mount, but it's worth a try. Maybe even loosen the engine mount bolts (top and bottom) and see if you can push the tranny sideways. If all that does not work, you could drill a hole in the side of the trans tunnel...

BTW, make sure the replacement speedo drive has the same number of teeth as the one you removed. If the teth are chewed up, then it was the wrong one in there. Then you need the number of teeth of the drive gear in the trans, and you will have to remove the trans to get to that.

[kiznarsh]

Yeah the more I think about it, removing the speedo drive wasn't meant to be done on the car. My plan is to buy a speedo drive for the same year that has 15 teeth, and that should allow it to correctly engage the drive gear inside the transmission. But if that gear is messed up, then I have bigger problems.

And dropping the transmission isn't something I can do on my own...

[ventura ace]

>
Working from memory I'd say there is no way to do this in the car. Likely not even if you unbolt the tranny mount, but it's worth a try. Maybe even loosen the engine mount bolts (top and bottom) and see if you can push the tranny sideways. If all that does not work, you could drill a hole in the side of the trans tunnel...

I have replaced the drive gear with the tranny and engine in the car, but it was not easy. I don't remmber the specifics, but I did a lot of loosening of things, probably dropped the drive shaft (which is easy to disconnect), probably disconnected rear tranny mount, lowered, tilted, and pushed everything as hard as I could to one side, to just barely get enough clearance to get the darn thing out and replace it.

Alvon

[kiznarsh]

I have replaced the drive gear with the tranny and engine in the car, but it was not easy. I don't remmber the specifics, but I did a lot of loosening of things, probably dropped the drive shaft (which is easy to disconnect), probably disconnected rear tranny mount, lowered, tilted, and pushed everything as hard as I could to one side, to just barely get enough clearance to get the darn thing out and replace it.

Yeah...no easy task to say the least!

Do you remember if the teeth were messed up on both the speedo drive gear and the gear inside the tranny? Or did you only replace the speedo drive and that fixed it?

I'm wondering what causes this...is one material harder than the other? Poor lubrication? I don't get the impression that this is terribly common.

[ventura ace]

Nothing was wrong with the gearing inside the transmission. The problem was with the right angle piece that you can remove from the outside. These are notorious for leaking or binding up from worn internal teeth at some point in their life.

Alvon