m40 speedometer drive gears

I had to recently replace the m40 trans in my '68 142s. I had to swap over the old speedometer drive gear and housing from the old trans to the new. But for some reason when I finally started to drive the car, the speedometer was off... WAY off.

Like a ration of 1.269:1 off. Meaning when the speedo says I'm driving 50mph, I'm actually doing roughly 64mph. 70mph on the speedo = 89mph IRL.

This also screws up the odometer, and since my fuel gauge hasn't worked in 4 years I need that odometer to read accurately so I can gauge my fuel consumption.

My point to this ramble is this:

Does anyone know what could cause this shift? The final gear is the same as it was (4.10) the only thing that changed was the gearbox.

On that note, what is the correct drive gear for the 4.1? What's the correct one for a 4.3? How can I tell which one is actually in there? (once I pull it out...)

21 tooth for 4.10

Jack said:

21 tooth for 4.10

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is there a color associated with it?

There are at least 3 different metal gears (the one on the shaft) for M40 trans. If the plastic gear is mated with the wrong metal gear it may ruin it.

A '68 140 with 4.10's should use a yellow speedometer gear with 16 teeth part #380164. What color was the gear you swapped?

The gear Jack is offering is for a M40 with 4.10's but it mates with a metal gear that's only used in '73-'75 boxes and I think the # of rev's per mile is different for the late speedo's.

You probably need to swap the metal gear from your old trans to get the speedo to be accurate.

hiperfauto said:

There are at least 3 different metal gears (the one on the shaft) for M40 trans. If the plastic gear is mated with the wrong metal gear it may ruin it.

A '68 140 with 4.10's should use a yellow speedometer gear with 16 teeth part #380164. What color was the gear you swapped?

The gear Jack is offering is for a M40 with 4.10's but it mates with a metal gear that's only used in '73-'75 boxes and I think the # of rev's per mile is different for the late speedo's.

You probably need to swap the metal gear from your old trans to get the speedo to be accurate.

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The '74 trans I picked up as a stop gap didn't have a speedo gear in it, so I swapped over what I think was a yellow, plastic drive gear from the '68 m40 box.

When I had the '68 box installed in the car it worked perfectly, but since I put the new one in, the speedometer, and by extension, the odo have been reading way off.

The 4.10 rear end I have is out of a '73 car. should I be looking for a metal drive gear to match the 73 rear end and the 74 tranny?

You already have the metal gear you need in your old trans.

You should be able to remove and install the gear without removing the trans. Remove the speedometer gear & housing, the rear flange, and seal and the metal gear should come out. You'll need to stick a screwdriver in the speedometer hole to slide it out. If your old trans is the original you may need to remove the rear seal housing to get the metal gear off. Early transmissions used a smaller rear bearing and seal and the gear won't fit past the casting.

hiperfauto said:

You already have the metal gear you need in your old trans.

You should be able to remove and install the gear without removing the trans. Remove the speedometer gear & housing, the rear flange, and seal and the metal gear should come out. You'll need to stick a screwdriver in the speedometer hole to slide it out. If your old trans is the original you may need to remove the rear seal housing to get the metal gear off. Early transmissions used a smaller rear bearing and seal and the gear won't fit past the casting.

Click to expand...

Hmmm I did transfer over the housing and the gear at the same time (the new trans had the housing hole welded shut) but I could swear that it wasn't the metal drive gear.. unless there are two different things, one that is at a 45? angle that meshes with the speedo cable and another metal gear on the inside of the trans I didn't notice.

I'll take a look at it hopefully today, but definitely this weekend

Put in a green gear and had a slight improvement. The ratio went from being 1.269:1 to 1.167:1
Still not 100% there but a step in the right direction.

I lack the tooling to get the metal gear off the old m40, and the trans is going to get changed in a little while anyway. So really I was just looking for a quick fix since my trip ODO is my fuel gauge.