Reduce DC voltage using a resistor..is there a specific reduction relationship?
From your description, I suspect that the closest arrangement, to what you are looking at, is something like the following (taken from Schwaller A. E., Total Automotive Technology, 2005, USA, New York: Thomas Delmar Learning.)



What you say you want is to subtract 15F from the reading. But this is really going to be a table of values used by the MCU to estimate temperatures. And, unfortunately, the value of the resistor used in the computer is unknown right now, as is the temperature to resistance table for the sensor unit (I'm assuming it is a two-wire device.) Finally, a simple subtraction of a fixed temperature is probably not so easy as the addition of some single resistor value. You may need to accept some kind of compromise here in order to gain simplicity. What kind of compromises are acceptable?

Can you collect some information about your system? Is it indeed a two-wire sensor system as shown in the chart above? (This seems to match what you are talking about... but it's best that we agree about it.) Also, can you find out any detailed specifications about your sensing unit? Would you be able to consider hooking up a \$4.7k\Omega\$ resistor (and perhaps several other values, too) to the location where the sensor goes and measure the voltage across it and report that information? (I'm assuming that with the sensor disconnected you read exactly \$5V\$ at the computer port for the sensor. But could you tell me, for sure, if that is indeed what you measure?)

Finally, if it is the case that you can consider the idea of adding another fan motor to your situation, you might think about the idea of tapping into your existing sensor (in parallel to the MCU), feeding the signal into a different controller unit. There are very cheap PID devices available on ebay, for example, which can run off of 12V, include relays, and where you can accept 0-5V input signals, provide a set point threshold for the new fan that is different from the MCU settings, and even provide hysteresis through the use of the P term in the controller (disabling the I and D terms.) I don't know if that has any interest, because I'm a little flummoxed by exactly what you are trying to achieve. But I thought I'd toss this one out there, as well, just in case it may matter.