Just some readings from a new Bosch Coolant Temp sensor (88 16v scirocco)
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degrees C kOhms -20 11.20 -10 10.00 9 3.36 20 2.42 30 1.72 40 1.16 60 0.72 69 0.63 74 0.59
At 25C is (2420 + 1720 ) *0.5= 2070 Ohm
so depending on your AREF you'll need 2063_256 or 2063_287 see EasyTherm