You want to be certain the thermostat is facing the correct direction. Redo it if necessary to be sure, it's the cost of DIY work. Sorry, no pics to provide. Temperature sender unit and single wire that goes to your temp gauge are not far from the thermostat and you could have affected [bad or loose] the connection when you worked on the thermostat which accounts for the lower reading. A failed temp compensator board in the instrument cluster will show a low reading but it is too much of a coincidence, although possible, that it failed when you replaced the thermostat.
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1988 245A. Past: 1979 245A; 1987 244 M47; 1971 164E