The stock transmissions in the older vehicles, E40D and gearing is what hurts them on mileage over 55 mph. They have a 4 speed automatic or 5 speed manual transmission. And most came with 4:10 gearing. They are great for low end power for towing but get hurt on mileage over about 55 mph. Mine got 18mpg stock when I cruised slow in the mountains. I've now done some mods to the exhaust and still waiting for results to see the improvements though.