Thermostat might have the temp stamped on it. If not, rig up a pan of water to boil. Hang the stat in the water so it does not touch the pan. A piece of wire, string, twist tie or whatever tied to a spatula across the top of the pan. Get a thermometer that reads in the 140 to 200 degree range and slowly heat the water until the stat opens.
Check your radiator. Since the engine is hotter at speed, it is suspect. Can you see junk in the coils inside? Also check the fins on the outside. Any places where you can't see through front to back? You can hose the outside to clean, but be careful not to bend the fins. And you might open some leaks. If you have to take it to a radiator shop, be prepared to have to buy a new core. The old one is likely rotten.
Temp range should be under 210. these were designed as a zero pressure system, and the boiling point should be 212. I usually consider the top of the practical temp range to be 190. Y'all don't say where you're from, is it desert or ice and snow? Makes a difference.
