so i filled up my tank and clocked my mileage. i got 210 miles on a tank and ended with 12 mpg. i was expecting closer to 20. i was wondering if this could be due to my tool box. im trying to clock it with out the tool box on this time. could it be the weight of my tool box. i have a diamond plate one. i saw a plastic one on craigslist and might buy if it could improve it. im would like to get a tune up since i doubt the truck has ever had one but funds dont allow it right now. so an tips other than modifying my driving which i have already done. thanks a lot for any help.