Pickup trucks have had mileage computers onboard for as long as manufacturers have known customers want to keep track of how much fuel they're using. Unfortunately, those computers aren't as accurate as you might think.
According to the Detroit News, there is usually a small difference between what the internal vehicle computer calculates and what you might calculate for yourself by keeping track of your miles driven and dividing that number by the number of gallons put into your tank at your next fill-up.
It's been our experience, with rare exceptions, that the truck's computer is optimistic about how many miles per gallon it is getting at any given moment. Additionally, if you tow or haul the computer could be adjusting for that in the overall calculation, which could also give you an unrealistically good readout. We've noticed that several truck makers are starting to offer specific adjustments to the trip computer that help the mileage calculator record and present a real-life fuel economy readout by allowing the driver to select what kind of work or towing is being done. This allows the computer to more accurately determine mileage and how many miles to empty.
Check out your own truck's computer setting to find out if you can make adjustments before a big trip so you can more accurately keep track of what your fuel costs might be. During our 2013 Light-Duty Challenge we found five out of the six tested trucks had computers that reported better fuel economy numbers than we calculated ourselves — one by as much as 12 percent. We assume that as truck computers get smarter and truck owners are allowed to input route-specific information these types of discrepancies will go away.