Finally got my truck on the road. The speedo reads WAY fast, at least 30mph over actual speed. According to the OEM sticker, it had a 8 3/8, 3:55 rear. I lifted one tire and spun it once while counting drive shaft turns. The shaft turned slightly less than 2 turns for 1 turn of the tire. Would that be about a 1:80 ratio? It runs very slow rpms at speed in O/D.
There is a tag in the rear that says 3150, but I don't know if it means anything.
The trucks fun to drive, and doesn't ride bad with the cut front springs and flipped rear axle.
The six can handle the ratio, it's hard to stall.

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'81 D150 2WD shortbed 4spd OD.

Count quarter-turns of the driveshaft while you turn the one lifted tire through _two_ revolutions, then divide your number of quarter-turns by four. There are enough ratios closely grouped that "about two" or "a little less than three" isn't accurate enough.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

OK, I'll try that. Thanks.

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'81 D150 2WD shortbed 4spd OD.