My car keeps over heating when I get on the freeway. It's fine on city streets. I put antifreeze in it. It seems like it runs out kind of fast but I havent noticed any leaking.
Overheating on the highway is likely due to a clog in the system, or a pinhole leak. Since your adding, I would say there is a leak somewhere. This leak is either going to be a hole in a hose, cracked radiator or leaking gasket (thermostat gasket, head gasket, etc.). If it is a head gasket, your exhaust will smoke a bit more than normal and will smell like coolant. I have had several leaks (radiator hose, crack in radiator) that I was only able to see after the engine was overheating. You want to look for signs of coolant that has sprayed and dried. Right when you get home from a drive, pop the hood and look for anything wet where coolant might had recently sprayed. You can also pressurize the coolant system with a radiator pressure tester and watch for leaks. Another thing you can try is to feel the radiator to see if there are cold spots. Cold spots mean there is a clog.