While we don't expect to see a Mad Max scenario anytime soon, where we'll be fighting for every precious drop of fuel, there are several good reasons for increasing the fuel capacity of your diesel pickup. With a larger tank under the bed, you can shop around for the best price on fuel and load up for a really long haul. (In fact, you can extend your range so much that you'll probably need to stop for human fuel a few times before your truck needs replenishing.) Having extra capacity is also a plus when towing increases your rate of fuel consumption.
Another advantage of an underbody tank is that it doesn't take up any precious space in the cargo bed, as is the case with an auxiliary tank. The latter also may require extra plumbing and valves for switching between two tanks.
Aero makes replacement tanks for every domestic make of truck and also tanks for toy haulers, RVs, and all sorts of custom applications. Instead of plastic (used by OEM vehicle manufacturers), Aero fabricates its underbody tanks from enamel-coated, aluminized steel with internal baffles, a drain plug, and through-bolted steel mounting tabs.
 Note the Aero tank's strong...  Note the Aero tank's strong steel mounting tabs that are through-bolted to the vehicle frame. All the necessary hardware and hoses are included with the Aero tank. |  |  Internal baffles add strength...  Internal baffles add strength and minimize sloshing of fuel within the tank. |
 Due to tight clearances around...  Due to tight clearances around our aftermarket Gear Vendors overdrive unit, the straps for the stock tank were cut with a die grinder. Even though it's a diesel fuel tank, we had multiple fire extinguishers on hand. |  The lower half of the original...  The lower half of the original fuel pickup assembly is reused in the new tank. The depth of the new tank must be verified with a tape measure so that the pickup will hang in the tank at the correct position. We were told the older fuel-level-sending units are known for wearing out in as little as 40,000 miles, so Aero replaced ours with a newer version made of a more durable material. |  Old and new-plastic versus...  Old and new-plastic versus steel tank. The tank capacity went from 36 gallons for the stock unit to nearly 60 gallons in the Aero underbody tank. Which one would you rather have under your truck? |
 All of the fittings, lines,...  All of the fittings, lines, and wires were removed from the old fuel pickup assembly and reinstalled on the new Aero mounting plate. |  The modified fuel pickup assembly...  The modified fuel pickup assembly was then carefully inserted into the Aero tank. |  The mounting plate must be...  The mounting plate must be seated properly in the top of the tank to ensure a good seal before the locking ring is tapped into place. |