Are What RPMS should a /6 be at while driving 55?

Since I found my car, I never drove it on the freeway. Matter of fact it didn't run at all when I found it sitting in the weeds in back of a bar.
It isn't the stock /6, I assume not the tranny either. It's a 80's motor I believe!??! At any rate I finally drove it on the freeway after I got it running and I swear it 55mph the motor was running at about 4,000RPM's! I have no tack to be sure, but it was running awfully high. I realize it is in part because it's a 3 speed Auto, and these days we are use to overdrive. Since the car was sitting for 2 years I was not real comfortable driving above 55, much less the gas I'm sure it was consuming.
Was this normal for a 73? Is that just how it was back then, or is something screwy? It felt like I was doing 55 in second gear. It sucked, but it was in third.
Any thoughts?
Thanks!