I recently put in an M46 (from a 3 speed auto) into my 1980 245, and now the speedometer reads about 10% faster than it should. The donor M46 came from a 1983 244 GL. I know that the small gear in the transmission housing on my M46 has a different number of teeth than the gear from my old transmission, thereby making the speedmeter read inaccurately, but my old gear wouldn't fit in the M46! I'm assuming my rear differential ratio is different than that of the 1983 donor car, and that is also part of the problem, but how do I fix this? Can I get a speedometer gear out of an M46 from a 1980 car with suposedly the same differential ratio that I have?

Jason 1980 245 B21F M46 141K (Dark Brown!)

Similar situation here - son's 81 has a rebuilt M46 installed by P.O. and it looks like it came from a 83-on car, as odometer and speedo read 13% high. The 83+ M46's had a 3.31 axle, son's 81 has a 3.73. That difference?...12.6%

I've pulled the driven gear and it is Black and has 20T. Volvo offer nothing bigger.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)

The 20 tooth gear in the car with the 3.31:1 axle makes sense because the higher rear end used that gear and the speedometer is geared differently. I think that is the case and I hope someone who knows will either confirm or correct this notion. I'm relying on memory and I could be mistaken.
I'm also pretty sure you need the 19 tooth gear for the 3.73.
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'80 DL 2 dr

Nope - being the driven gear, the more teeth on it, the slower the cable - I need either a 22T (10% slower cable) or 23T (15% slower) to solve my 13% too-fast problem. Since the 20T is the most offered by Volvo, they must have changed the pitch of the driving gear on the OD output shaft. Both our 81 (3.73 axle) and 83 (3.31) speedo's are 85MPH units with the code "r 0,980", so one would think their internal gearing is identical.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)

Hmmm... my speedo has "r 0,980" printed on it, I have a 3.91 rear axle, M46 and 20 tooth driven gear and I would swear it's correct.

The fact that Volvo doesn't offer more than 20 teeth on a driven gear makes me think your problem lies elsewhere. But the only other variable is tires. Unless, as you speculate there is some difference in the drive gear. But the way Volvo seemed to cling to certain designs makes me think not.

This is one of the more confusing 240 topics, for me anyway.
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'80 DL 2 dr

Agreed! It's possible that in the original design of the trans/OD output shaft speedo driving gear and cable driven gear they didn't allow room for more than 20T on the driven gear, as the higher (lower numerically) axle ratios weren't dreamt of. So when the 3.31 fuel economy oriented ratio was deemed necessary, they may have been forced to change the pitch of the driving worm gear on the tailshaft - a much more expensive mod than the little plastic driven gear. Who knows?
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)

I also had to change the number of teeth on the gear in my odometer. I went from a 14 tooth to a 17 tooth. My odometer was 25% high. That plus the new gear in the tranny and my odo is now accurate.
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Simon 80 240 300k 17 years. 'White Lightning'

You can get the axle ratio from a plate located on the back on the drivers side of the axle. use the previous post to get the correct teeth. Your local Volvo parts store will have the gears.
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Simon 80 240 300k 17 years. 'White Lightning'

The wagon originally had an auto trans, correct? I believe that it should have a 3.73:1 rear axle (or would have had one originally), which, with a manual trans, needs a 19 tooth gear. Find out what the ratio is for sure.

PROJECT 242 .COM sez:
3.54:1 uses #380754, 18Teeth - blue
3.73:1 uses #381681, 19Teeth - red
3.91:1 uses #380753, 20Teeth - black

Don't rely on the color though. Count the teeth. I have a 20 tooth gear in my O.D. that is tan in color.
Also, the gear from an M45 is different than in the M46. Well, the gear is the same but the rest of the apparatus is different and they won't interchange. At least in my limited experience in this matter.
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'80 DL 2 dr