Have you had the exhaust gases checked at various engine loads, have you tried moving the base engine timing, mass airflow sensor(just because it meets specs doesn't mean it is working properly), brain transplant? My brother-inlaws '92 5.0L 5 speed long box got 22mpg on the highway. 5 different '92 5.0L 150s, long boxes got 20-22 mpg, short boxes got 15-18 mpg regardless of trannys, all 4X4s. Obviously something in the air/fuel mixture management system is 1. sending an inaccurate signal, because it was replaced with wrong one/factory goof, not monitored by brain. 2. Brain has bad programing. What you need is a mechanic that understands the reason for and function of each component and how they effect the brains desicion making process. Have you checked your fuel pressure?