ISS/All:
Check out this very long thread from several years back on this very subject.
https://sprinter-source.com/forum/showthread.php?t=24859&highlight=rpm+calculations&page=5
It goes on for 48 posts, and the calculations (and calculation spreadsheets) are in there.
The things you need to know to get the best possible accuracy/result:
1/ Tire revolutions per mile (this is available on Tire Rack and the various tire manufacturer's websites).
2/ Your rear differential ratio.
3/ Your final drive ratio (considering that 5th gear is 0.83:1 overdrive) (must be subtracted from your rear differential ratio).
4/ The 5% torque converter slip (it does not lock up 1:1 in 5G-Tronic/NAG-1 transmissions)!
5/ Your actual speed (from a GPS or perhaps a Scan Gauge II) as your speedometer will read a couple (to about +5) mph higher
than your actual speed.
ISS:
OK, now we know you have the middle ratio (3.923:1) of the normal ratios available in USA/CAN Sprinters.
In 5th gear you must reduce that ratio by the 0.83:1 overdrive ratio (5th gear) so actual 5th gear ratio is 3.923 x 0.83= 3.256:1.
If you want the actual 1:1 ratio (through your transmission) you would need to manually downshift to 4th gear.
I'll let you read through the previous link and make your own calculations.
Without an OBD-II device to give you accurate digital RPM (and possibly correct your speedometer) it's going to be very
difficult (and somewhat less than accurate) to assign numbers reading from analog gauges.
And, last but not least,
WHY are we calculating all this? What is your desired outcome here?
If you get the Scan Gauge II, you will have all the accurate numbers, but if you really want to get the best
fuel mileage and performance from your Sprinter, set the SGII on % engine Load and drive your Sprinter in a
manner that keeps the % engine load as low as possible as much of the time as possible by manually downshifting
your transmission when needed to keep the % engine load @ < 80% as much of the time as possible.
Hope this helps,
Roger