About 2 years ago, I made a trip from Florida to Pigeon Forge Tn. Roughly 5 hours on I 75 at about 70 mph. Rest of the trip using state and local roads, some of which were two lane and 30-35 mph limit through the state/national park/mountains. Total round trip mileage about 1100 miles. Averaged just a tick over 24 mpg, and in the low speed areas didn't even get to converter lockup for several miles, at a time. In around town driving get about 16-18 mpg. Combined driving from home to town, and around town, 19-20. Fillup yesterday was 20.1 mpg (292 miles). Vehicle is a 88 Diplomat 318 cid. bought new and has 132,000 on the clock. Several years ago, I disconnected the lean burn, and replaced the distributer and carb, with earlier models (can't remember the original application), using a GM HEI module and coil. I have a O2 senser, and have rejetted the carb, and recurved the distributor.