#1 confirm the actual error with a GPS or interstate mile markers (go 5 miles/5 markers) for the odo & should go (1) mile in 1 minute with the speedo dead on 60 MPH for the speedo & usually when the speedo reads correct the odo will also but if not the speedo head would need to be sent to a speedometer specialty shop tho some people have successfully tweaked em at home. get the right speedo gear in there to correct the odo then see if the speedometer reads MPH correct. there's a bunch for sale on eBay & FABO, not cheap tho but it is important to get it dead on. EDIT meant to say that usually when the odo reads correct the speedo does also

Pretty much guarantee that adapter won't fit the early TF setup.

Notice that in the early setup the adaoter is part of the speedometer cable itself.

A 1964-65 B-BODY, 1963-65 CHRYSLER/IMPERIAL cable pictured, A body similar

Also a totally different design.

Interstate highway mile markers are a pretty reliable way to judge the offset. Sometimes state routes (2 lane black top) will also have them. The thing I did many moons ago: grab a car I knew read correctly, drive it about 10 miles, turn around. Note what it read out / back / combined. Take the exact same run in the one I knew didn't (tire, axle change) then calculate the offset. Put the tail on ramps to keep the fluid in the trans and swap gears then re-run the loop. As a final sanity check, I go looking for one of those road side construction zone things that flash your speed at you.

You can put an app on your phone that will tell you you speed continuously, they are pretty accurate and easier than any other method I seen listed.

They can split the cable and install the necessary fittings to place it anywhere between the cable ends

That gets beyond just buying a ratio adapter, particularly when it would be substantially cheaper and easier to just get the right pinion.

On a side note on the odo & speedo, if one of em is inaccurate I'd rather have the speedo correct. tickets ain't cheap!

Here's the thing with that: some of us were swapping gears before smart phones existed and old habits die hard.

True, but around here finding a mile marker is harder than finding gold at the end of a rainbow.

Here's what the early reduction gear looks like.

GEEZ!!! just offering the guy an option, it's his decision, not either one of ours