I have a 2016 Sonata and when it is to be over 80 degrees outside the car will want to over heat. So I'm old school and I turn the heater on to get the temperature down and it works but now if I run it at 70 to 80 mph its starts to heat up again. So I will slow it down to 60 mph and it goes back in temp. I would think the faster you go the more air flow it gets and cooler it would run. Not this car. If its the thermostat wouldn't it throw a code???? Coolant is full. Could it be just the cluster is messed up??? Not sure where to go from here. Oh and the fan kicks on like it suppose to.