No no no, 3000-4000 rpms is the average speed for highway traveling, driving too close to the red mark on your rpm is bad
the rpms should rise and then drop no more then 500 rpms when the fan on the radiator turns on and then off.
If your 1992 Voyager does not seem to drop into the proper RPMs so that you are running at 5000 RPMs at 65 miles per hour, the timing belt might need adjusted. It can also indicate the idle is set too high.
Certainly. It's been done for years before overdrive became a thing... it just means you'll turn higher RPMs at speed, and you'll want to drive slower if you're conscious about fuel consumption. I had no problems when I took my 87 Silverado with a Muncie 465 direct drive transmission on the highway.
in relationship to engine rpms a ratio of something like 3.08 would have less acceleration and lower highway rpms compared to 4.11s. the 4.11 ratio would give you better acceleration and less top speed and lower gas mileage at highway speed
If the rpms are to high for to long the engin will over heat and you will drop it
Try replacing the map sensor
If you mean why the difference in RPMs between idling and driving, the difference is due to the engine load; At idle, the engine isn't really doing much other than staying running, but under load (driving) the engine has to put its power out to the drivetrain, hence the symptomatic loss of engine speed (RPMs).
First make sure all your tires have the same amount of pressure about 35psi/40psi(22's). Also city driving is the worst try to get more highway & keep your RPMs under 3(3000)!
The A/C puts an extra load on the engine, so it's natural that the engine RPMs drop slightly when the A/C is activated.