There ought to be a tag on the driver's side of the diff, the middle no. is the ratio. The non-od diff is probably a 4.10:1, the od diff... I don't know. Maybe a 4.30:1. you can raise the rear wheels, and turn one complete revolution by hand. Have a way of counting the revolutions of the driveshaft, that will give you an approxamation of the ratio.

You want a lower no. for better economy and maintaining contemporary traveling speeds. 4.30 is okay, 4.10 is good, 3.90 is great.

that's how I understand it anyway. I'm actually really happy with my 4.30 in the '70, the other cars have/had 4.10s but have other issues that effect economy. None have OD.

Cheers, -Sean
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'66 122s, '70 142s, '74 142e... Blue is beautiful.