good day to everyone. mga sirs, i hope you can help me with my headache

. my car, a manual transmission, diesel fed, toyota innova 2014, as i have observed, lost some speed or power. i dont know which is which (speed or power). explanation: 3 months ago, it used to give me a good speed at 2,000 rpm.
at gear 1 it gives me 20km/h
gear 2 --> 40km/h
gear 3 --> 60k/h
gear 4 --> 80km/h
gear 5 --> 100km/h
but now it gives me lesser speed for the same RPM (2,000rpm)
gear 1 gives me 16km/h
gear 2 --> 34km/h
gear 3 --> 54 k/h or less
gear 4 --> 80km/h
gear 5 --> 90km/h
in short, there is around 20-25percent speed reduction or power reduction regardless of the RPM (either 2,000 or 3,000rpm). i dont know which is which. i have asked mechanics and they gave me different suggestions - all of which never rectified the problem.
oil change
gear oil change
fuel additives
engine air filter
engine oil filter
fuel filter
all done but the problem still persist

. can you please enlighten me on what could be the problem?
thanks in advance po sa sinumang sasagot at talagang masakit sa ulo isipin .