into an actual change of speed by 20 MPH (give or take). Is that the case. As well, 80 MPH is above the speed limit in the US so I'd wonder where you are doing your driving. If you've no sense of speed change, I'd suspect your gauges are reading incorrectly. It could be that the problem lies in your tach or the sending unit. At 20+ years old, I have seen gauges get flaky due to corrosion of electrical connections.
I just recently got a new problem with my car. It's a 1993 Nissan Patrol/Safari/Maverick (depending on where you live.)
It's carbeurated and manual transmission.
The problem is that when I'm on the highway cruising at a constant speed of let's say 80mph, the engine RPM will randomly increase by about 1000 and then stabilize again. This will happen randomly but it's mostly when I'm in the 5th gear and cruising.
It happens on level ground and the car speed stays constant with no changes.
Any ideas? Before this happened I was a bit rough on it powershifting and power sliding (not sure if that's the correct term in English, but I was doing something similar to donuts) it in the desert.