Lugging an automatic

[Koush]

Okay so i just got my driving license a few months ago and my dad gave me his*13' Mazda 3. He had taken care of it so well and i have a feeling im ruining it.

It is an automatic with manual shiftability. Sometimes when im passing people i like to put the car on manual and floor it just so it wont downshift.. i stay around 3000 to 4000 rpms on 4th gear. I know im consuming the same amount of fuel, or even more, than just flooring it on drive but i thought it puts lesser stress on the tranny and axles (i blew both my axle boots btw).

I recently came across this forum that said about engine lugging and i wasnt too sure. Also i drive at around 4500 to 5000 rpm on freeways for staright 2 to 3 minutes at like 160 to 180 kmph..i dont know if its really that bad for my engine

*So i just wanted to confirm with people who know cars better than me. Any sort of support is appreciated. Thanks*