The simplest way to check is get a cheap optical/photo tachometer.
All you need to do it point it at a spinning object that has one reflective or bright marking on one spot , and read the numbers.
If the numbers don't match, then it's the tach that's off.
I've done this math in other threads, so here goes
With 3.08 diffs, 1.9 hub gearing, and .75 overdrive, at 55 mph and 37" tires you should be turning right at 2200 (2190) rpms.
(the tires don't actually matter in any of the math here because the tach and speedo both get their readings before them in the system)
In the threads where the tachometer being off due to having the smaller pulley on the 400 amp generator, the usual discrepancy is instead of idling at 700rpms, the engine is reading 950rpms That puts it at approx 1.35-1 difference.
Another posted that the tach reading 2000 rpms compared to an optical tachometer on the harmonic balancer/crankshaft showing 1500 rpms, comes up with a 1.33-1 difference.
Going by the above 55mph being 2190 rpms, multiplying that by 1.33 gives you 2920 rpms at 55mph, which is pretty darned close to the numbers you are saying, and lines up pretty well with the other threads discussing this.
I have a 2006 M1165, 6.5 turbo, 60mph speedo and a tachometer. My problem is that the tach rpm reading is wrong. The tach shows idle at 950rpm, but my handheld rpm meter shows 700rpm. And with the engine speed set and showing as 2000rpm on the tach, my handheld meter is reading 1500rpm. I...
steelsoldiers.com
Did any of the m1152’s come with really low gearing? I’m having trouble with my new 52, I’m at 3300rpms at 63-65 when multiple other people say theirs is 2700-2900 at 70-75. I believe I feel lockup, either way, it wouldn’t drop that much. I have shifted from 3rd to 4th to make sure it actually...
www.steelsoldiers.com