Odometer underreading by 50%

Hello,

Have read through odometer posts and didn't see this exact condition; am reading through the Jaeger article by Anthony Rhodes as I work up to opening up the speedo case and counting gear teeth.

Thought I would try here first- has anyone come across a odometer/ speedo setup that reads accurately in MPH but is underreporting odometer mileage traveled by exactly half? I thought I had some timing and carb issues which are now solved, because my mileage was reading in the 4.2 to 4.5 mpg range (1972 Series III 88" petrol). Rebuilt the rochester carb, checked comperession (8:1), reset timing and she runs great. In fact better than ever. Except only increased to 5.7 mpg. (I only checked because the gas gauge conked out so am writing down where I stand to avoid running out of fuel- used to do the same on a plymouth valiant!).

My wife suggested running a circuit through town and comparing the mileage which I did. She was right of course. Math was so accurate to be 50% under with a 3 decimal place rounding error. I'm getting around 11-12 mpg in actuality. I'm happy to proceed with writing down mileage in my book and recalculating, but since I'm tackling various issues, though I would see if I can solve this next. Anyone ever see something similar? Of note is this land rover was put back together from 2-different ones and I have another (2) odometers sitting on the shelf from the prior owner.

Thanks for any ideas!