Are you assuming that the 20yo temp system is still OK? This issue keeps coming up and coming up and coming up. Go get yourself a digital laser thermometer (or borrow one from your mechanic). Aim it at the base of the temp sender and compare that to the reading at your gauge.

The following is from my engineer/geek buddy who works on satellites at Boeing (formally Hughes). The temperature of the wire between the gauge and the sending unit will change the resistance of the wire.

Back in the archives there is a table of resistance/readings from a gauge. A 1 ohm change in resistance will change the gauge reading fairly significantly (5 degrees? I don't exactly remember). So you can see if the wiring in the temp reading system changes in temperature, the resistance changes and the gauge will not read accurately.

I don't think your gauge is that far off and the 5 or 10 degree difference between different cars seems reasonable to me. Until you develop a base of comparison, you can't really judge whether your system is reading accurately or not.

I know its unnerving to read a high temp gauge, but maybe it isn't high to start with...........

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)