Joe h's recomendation is a good one. will the truck go ahead and boil water out if you just leave it sit and idle? If not theres a good posibality the gauge is reading wrong, because you have lost some ground if its an electric. if its a regular manual type gauge with a bulb look for capilary tube running too close to exhaust. Good idea to make absolutely sure you have a problem and its not just the gauge pulling your leg.
