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.
While do-it-yourselfers can do a tank swap if they have the right tools, most of Aero's customers have the installation done at the factory. That may be because the warranty on a factory job is slightly better-three years and 50,000 miles versus two years and 30,000 miles on a DIY install.
Our particular application on a '96 Dodge Ram required some custom work in order to clear the mounting plate for a Gear Vendors gear-splitter. For a modest extra charge, Aero can custom-weld a tank with a recessed area to make room. Even with this modification, the tank capacity went from 36 gallons for the stock unit to nearly 60 gallons in the underbody tank. The 12-valve Cummins in this truck typically consumes 19 mpg on the highway, so that's more than a 1,000-mile range between fill-ups.
Here's one other point to consider when changing out the tank: the sending unit on the fuel pickup assembly on older Dodges is known for wearing out in as little as 40,000 miles, and that was the case on this truck. Remembering to reset the trip odometer at fill-up time (in order to estimate the fuel level by miles traveled) was getting annoying, so we were really glad that Jake Jacobs at Aero swapped out the sending unit with a newer type made of a more durable material.
Jacobs points out one difference in the gauge readout after upgrading to a larger tank. The fuel needle will stay on full for the first 15-20 gallons because Aero uses the factory fuel pickup and does not make any adjustments to this part. But look at it this way, even if you're not actually improving your fuel efficiency, it at least seems like you're getting more miles to the gallon!
