is it ok to shift to 6th at a low speed and drive at like, 1800 rpms? i guess it would save gas... but is that hard on the car? is there a minimum rpm that you should drive at? help me out

 

I think driving at a steady state in 6th @ low rpm is OK provided you are not trying to go up a hill or accelerating hard. You don't want to put the engine under undue heavy load like these conditions. This is called "lugging" and no engine likes that.