Diesel car

I recently failed my driving exam. The examiner stopped the test in the middle and told me that I was driving horribly wrong.
It was a diesel car and when I tried to drive on 2nd or 3rd gear on a 30 speed limit road the rpms drop to 1000 .
The rule says that you should keep the rpms above 1300.
What do you suggest? How to drive a diesel car in economically?
Should I ignore rpms and drive in higher gears anyways?

Ps. I told her that i am trying to keep the rpms above 1300 and if i drive in higher gears the rpms are dropping down. But she didn’t gave me any clear answer and simply said that the first gear is for only starting the car.

There is no 1300rpm rule. It’s different with different cars. Diesel cars generally have more torque so they can handle lower rpms better. So do some gasoline cars. My Mazda 3 does fine around 1000rpm or even lower for example while my friends VW Golf struggles. Both gasoline cars.

If possible try to focus on the engine sound instead. It will tell you if it struggles.