I will get a reading of 3156 inches which I know is actually 3156mm's and is what I had drawn. If I then set it the Units to my desired mm's then the reading changes to 80162.55 mm?!!
3156mm x 25.4 = 80162.55 mm (calculator actually answers 80162.4 mm)
So the key issue is a setting of 25.4mm = 1 inch.
In your case .... The drawing is being measured by a scale value of x25.4 ..... or ..... The actual Model Space is set to a Drawing scale of 25.4
Select the "Orthogonal" dimension tool / RM Click / Properties / Units,Tolerances
At the top >> Append Units << un-tick.
Is there a setting of 25.4 << Change that?
Look at all settings, Compare to how they should look / set-up, when using a Normal Metric Drawing.
Did the drawing go through a DWG conversion? .... This could could explain the "inch" issue ... as DWG is Imperial by default.
Re check settings via Options / Space - Units
Sometimes its easier to just create a New Blank File from Page Setup Wizard .... Using the Top line only >> EDIT / Select all / Copy >>> Then in the blank new file "Paste".
Save the New file .... and see how it goes.