What is a good engine speed range for highway driving?

My car only has four gears, cruises nicely from 80-100 kpm before reaching over 2000rpm. I'd like to know what is the upper most engine speed (rpm) that my motor should stay under while driving whilst keeping the engine healthy not overworking. I have a 3.0 v6 Honda VTEC motor.

I'm guessing that your engine has a built-in rev limiter to prevent it from spinning too fast. Even so, I would try to keep it below 6000 for normal driving.

It's used I don't have the owner's manual.

The uppermost is 12000 or so and then the engine should blow up. Read your owner's manual as they have recommendations in there.(FROM THE MANUFACTURER)

Totally depends on the engine.
Where the torque is and how high the rev.
I know cars like mine that cruise at 110KpH at less than 2000 RPM.
I know cars that cruise at 110KpH at around 10,000 RPM. (They rev to 18,000 and have a very high power band.).
Most normal petrol engine rev to over 6000RPM.
My diesel revs to 5500 RPM.