Eco driving

Winton,

That's very strange for a driving instructor to ask the student to stay in 3rd gear at 30mph. Usually you should change to fourth gear when driving over 15mph. Like you said, this saves petrol and is better for the environment because of the low revolutions of the engine motor.

You should probably ask your grand-daughter what else her instructor has been teaching her just in case she learns doing things wrong.

This depends both on the circumstances (road conditions, driver intentions in the next 15 seconds or so, etc) and the car itself. For instance, in my small petrol engined tuition car, I encourage my learners to consider being in 4th at around 25mph, if in an urban 30mph limit with few obstructions and good views. However, in my medium sized diesel, I will often be in 3rd in those circumstances.
Incidentally, using 4th at 15mph will stall many cars, and will certainly result in severe inflexibilty from the engine. I would also expect that to result in a minor driver error on test in many instances.