Years ago, I put an M46 from a 1983 244 into my 1980 242 to replace the original M45 with the bad input shaft bearing.

The donor car had the 3.31 rear and the 1980 has the 3.91. The speedo has been wrong ever since. I was expecting this to be the case, and figured I would get around to putting in the correct gear someday.

Well, someday is here and I have a problem.

Everything I can find states that an m46 with a 3.91 uses the 20 tooth gear. But, it already has a 20 tooth gear in it from the 83 with 3.31.

The speedo is off by a percentage and not a fixed amount, so it would have to be a drive ratio issue or incorrect speedometer(?).

Which gear do I need?

BTW, I am using the R-sport cluster but still have a couple stock clusters available.

I too replaced the 3.91 with a 3.31 in my '80 245 with M46. Still haven't fixed the problem. Also increased tire size to 205/75r14 for winters but still have 195/75r14 all seasons. I am planning to go up to 205/75r14 all seasons (used on older Dodge Caravans).

There are two gears. The one in the transmission and the one in the speedometer. It's the gear in the speedo that has to be changed. But the worm gear that it drives may also have to be changed.

As you say it is "off by a percentage". Get a GPS and find your actual speeds and figure the percentage difference. And drive accordingly.

I was thinking of trying to print out a speedo overlay. Or ordering one from someplace that makes replacements in different colors. Haven't done either yet.

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1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.

I will be swapping my M45 for an M46 in my '78 242. Will also be swapping from a 3.91 (stock) to a 3.73 rear end ratio.

What kind of work am I looking at here - should I be swapping the worm gear in the trans while I have it out? Direct me to a thread where all of this is laid out (if it exists :))

Which gear do I need?

The 20 tooth gear.


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'80 DL 2 door, '89 DL Wagon