1983 Plymouth Scamp GT - Lacking in Performance

My baby has been maintained over the years and has most engine components replaced after they fail one by one. What is irking me are some performace issues that have been starting to come to light. For starters, Something seems wrong with my gear ratio. (The speedo is disconnected but...) I can get to about 35-40 in 3rd gear before going over 3Krpm. 4th gear is anywhere from 35-50, and 5th and final gear gets me to about 65 max. I'm always in the right lane on the highway. Really doesn't go much faster than that, though i think i hit 70 once, downhill, in neutral.

That was during the return part of a one-hour drive on some decent grades through the mountains. Then the check engine light came on. After a cool down period of about a half hour it has been fine. Am I really looking at a permanent top speed of 65? Are there any tricks or tips in terms of maintenance or repairs that could help this out?

When jiffy lube forgot to put the oil cap back on my GF's 98 isuzu rodeo, there was a period where she was having to shift into higher and higher gears than usual to maintain average city speeds. It was only after this negligent error that her car started to remind me of my own...


I keep the fluids changed and filled regularly, and all the filters too.
If there's nothing I can do about this, anyone know how to tighten a parking break so I don't roll out the driveway?

Thanks in advance!
-Portnoy