Don't even need to rely on the tach.......1,000 rpm difference is very noticible....

Its not just a little different...its a lot different at the same road speed. And its noticible without even looking at the speedo either if you drove both.

Wonder where this is going to go if the rear is a 3.07? Only parellel that I can think of is my two 240ds both identical. One at 70 sounds like a threshing machine out of control while the other at that speed sounds normal. Or normal for a 240 as comparison with other cars is hard if not impossible. :D This should get interesting. Now not so sure about your glo plugs. :D

If you try to make a test with both wheels off the ground, the drive shaft has a tendency to remain stationary and the wheel that you are not turning will attempt to spin in the opposite direction, so, it's not usually a valid test.

I would expect the 300SD to have lower gears because it weighs more then the W123. That might make a difference, I bet the trans are geared different.

How???
 

How would high gear on both trans be different? They are both 1:1 ratio

For example I know a 420SEL has lower gear ratios then my 300SDL. For example the 420 can carry third to 120mph and mine can only carry it to 83. The V8 has a lot more power so it doesn't need the rpms to pull a heavy car.

I am thinking the same thing, the W116 must have a solid 500lbs on a W123 so MB wants it to spin higher so the 617 can pull it along.

Point is the differentials on BOTH are stamped 3.07 and the RPMS are dramaticly different on both..has nothing to do with smoothness as both are fairly smooth..didn't get a chance to check that tonight (check revolutions of wheel VS driveshaft...had lot of yardwork to do as we are getting rain tomorrow.

Problem with both wheels of the ground is the one with less drag is going to turn more.....works better with a limited slip differential............well it the rain holds off tomorrow I will check it then or if it stops for a while.

The difference you mention is in the spread of the gear ratios from 1st, 2nd, 3rd etc to Top gear (4th, 5th, 6th, whatever) which is a 1:1 ratio UNLESS the transmission is equipped with an OVERDRIVE.

To my knowledge, manual transmissions have not had an OVERdrive for a lot of years, and automatics never had them. 'Overdrive' in an automatic is usually another word for locking torque converter, for a 1:1 ratio without slip.

Best Regards,
Jim