Very low gearing?

I got a 99 ram 2500 Frankenstein and it seems to be super low geared. It's a 5.9 cummins, auto transmission, tires are stock size. It is a gas v10 Vin so the Vin number is useless. There is no gear marking on the diffs. 4th gear runs at 40 mph at 2k rpm. Overdrive runs 60mph at 2k rpm. Is this a 4:10 rear end? I've had a lot of these but they never ran that high of rpms. I don't usually mind it but when pulling heavy and my instinct says to turn Overdrive off and then I'll only go 40mph because I don't like to run the engine much faster than 2k.

I guess my 2 questions are is it a 4:10 or did it get a different rear end ratio when it was frankenstiened together, and with it being geared low will using Overdrive hurt anything? 10k towing is usually where I turn Overdrive off.

You can calculate gear ratio with tire size, trans od ratio and rpms. Plenty of formulas for this online.

Thank you for the response, however it didn't really answer my question. I don't know the ratio for overdrive so that makes the formulas useless.

Using the formulas, assuming 4th gear is a 1 to 1 it says I have a 4.70 rear end. I'm wanting a comparison from someone that has a 2nd gen that know what rpms their pickup runs at in OD