My car is the same year as yours and with the same rear axle. I believe 3.91:1 was all they put in the standard trans models that year. I replaced the 4 speed gearbox with an M46 from an '84 last summer and this subject was of great interest. Of course, I wanted my speedometer/odometer to be correct. I noted that the gear assembly in the M45 was not interchangable with the one in the M46. The gear was identical, except for color, but the rest of the apparatus was different. They both had 20 teeth. I figured everything must be okay.
Then I noticed in the '84 Volvo owner's manual that they do not include the 3.91 axle for that model year. That sort of threw me. Then I found some information at Project 242 that reassured me I was using the correct gear.
I have been wondering which gear was common to two ratios. So I guess the 3.31's must have the different speedometer.
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'80 DL 2 dr