sportage overheating

I have a Kia Sportage that is overheating. It cools down when traveling on freeways or high speeds. The fan is running but I'm not sure if is running at the correct RPM. Does anyone know how to check this? The thermostat has been changed.

Unless you have a blown head gasket..which would then not allow you to cool down on the highway at high speed...most usual problem is failed fan clutch, a common issue. WHen it DOES engage, one cannot miss the whooshing sound that results: some even think there is a problem when this happens. So if you do not feel like hearing a loud windstream, it probably does not rotate at full speed.
Does turning the A/C off alleviate the problem?

is the condenser fan working? Also a common problem on Sportages. Turn A/C on and look at it is the electric one on the front of the vehicle. Is it on? no replace it Is it on? yes. If the fan is working and it looks like the fan clutch is working. Try this start your truck and watch the temp gauge. Does stepping on the brake pedal make the needle move? Does turning the head lamps on make the needle move? If they make the needle move the vehicle may not be overheating at all it could simply be a ground issue