I have a 1993 F150 xlt 4x4 with dual tanks. I think it's got a.vacuum leak, but it's been driving reasonably well. I've been getting around 8-10 mpg. Filled up tank yesterday and drove home. Total driving miles about 28. Problem is this: I just looked at the fuel gauge, and it was sitting on empty. Switched to aft fuel tank and it was ok. Any ideas?