Hello 540itouring,
Welcome to the forum.
I would disagree with the blanket statement " car does not need cooling fans over 35 mph . . . ", to the extent of under NORMAL conditions. It will need them when you are following too close and probably will need them when climbing a long hill .
I have had cars with electric fans which experienced failures and they would heat under those conditions, with a/c on. It would be a good project I suppose if you allowed a high temp override to enable the fans. First thing you should do is make a flow chart on paper of what your program needs to do, and in what order, decide how many I/Os it needs to control. To measure temp, do you want a sensor or simply a switch, that will determine if you need analog functions. You want to count pulses from the speed sensor or tach. or abs sensor so you will probably need a timer, likely a 16 bit timer. Those are my immediate thoughts.