I believe you have two possible failures, first a bad MOSFET which I doubt, second you are not fully enhancing the MOSFET. I would suggest you place the 10K resistor from D10 to the source and something in the few hundred ohm range between D10 and the gate. I believe you will then find your calculations right on! Be sure all the grounds are connected together including the source.
Yes, something is wrong. But you failed to post your circuit and your code so we have no idea what yet....
BTW did you source your MOSFET from a reputable source? If not its likely to be counterfeit, pretty much all random sources of semiconductors on eBay etc these days are counterfeit as the genuine sources are undercut by the criminals.
Attached is the simple schematic on how the Mosfet is connected. Kindly enlighten if there's any mistake.
The load is consuming 1.6 ampere @ 9 Vdc; measured using a multimeter. And I have a Ruideng AT35 tester too.
I have measured the voltage values;
Gate-drain voltage is 4.8 Vdc
Drain-source voltage is 50 mVdc when cold and 60 mVdc when above 50 degC
Gate-source voltage is 4.8 Vdc
That gives 0.0375 Ω which is equivalent to 37.5 mΩ and it is relatively higher than the built specification. However, at these values, the power dissipation should be still well below 65 degC; theoretically is 6 degC + ambient temperature.
I measured the voltage values directly on the Mosfet legs when it is on load.