86 stang overheating

Hi there I have an 86 Mustang 6 cylinder that overheats on the highway, if I am only driving in the city no problem never goes over half but on highway it will redline, I put in a new 160 degree thermostat just to see no change any ideas?

How's the coolant look? Is it full of rust? You should consider flushing it if you didn't when you changed the t-stat. If it's clean, than it may be a an air pocket in the top of the engine where the sensor is located, or a lack of flow through the radiator or other orifice.

Also check your water pump, it may not be working very efficiently anymore at the lower RPMs at highway speeds.