I was driving on interstate at 70-80 mph with the a/c on for over 4 hours and noticed that for the most part, the tachometer was showing that the engine was running at 3000rpm. Is this normal or on the higher side? This rpm was on the straight road and no elevation or slope.

Also, another question suddenly crossed my mind that was bugging me for a long time during the drive. When I am driving at any speed, what happens if I shift the car in neutral and turn the ignition off? First of all, can this be done or is there any lock in the system (something like, if the car is going at even 10 mph, it can't be shifted into reverse gear)?

Thanks

 

Yes that RPM is normal for those conditions.

You can shift into neutral no problem. You can shut it off too but it would be a regrettable decision. With the ignition off you would lose things such as power steering and brake booster. Then if you turned the wheel the wheel would lock in place.

 

And if you shift to 1st gear at that speed your clutch disc can explode even if you don't let off on the clutch. Manual transmission follow instructions even if it not in their best interest.