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re: "...For example, at 60 MPH in 5th gear, engine speed used to be 2100 RPM but it is now 2300 RPM...."
5th gear? So it's a manual transmission (automatics only have 4 forward speeds)! That precludes a possibility of Torque Converter slippage because there is no TC. And that means that, when you're in gear, your engine crankshaft is firmly linked to your wheels' rotation (unless your clutch is literally melting :-).
So assuming that this is based on your instrument panel speedometer, there is no way that you can have a road speed:engine rpm ratio change as you described. And this is regardless of cooling system work (or even tire changes, since your displayed speed is based on wheel rotation, not tire circumference). Only some electronic defect developing in your '93's speedometer circuit board might possibly explain this.
If you're basing your speed on a GPS (Garmin, etc.) which tells you road speed, however, then there are factors involved that could indicate this change, including the occasionally awkward position of satellites (called the ephemeris), etc. But these would be temporary.
Also, a change in tire circumference might show up as a change in GPS-based road speed.
I would suggest that the precision (or lack of) with which you examine your speedometer display is likely the blame.
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