GM Dually Fuel Tank Upgrade

Updated Feb 1, 2014

GM Dually Project


56-gallon Transfer Flow fuel tank replacement and F.A.S.S. filter system a perfect long-distance setup


By Bruce W. Smith

Fuel. It dictates how far we go and when we need to stop. On many days it seems there’s plenty to get the job done, and on others, when time is pressing, the fuel gauge seems to fall way too quickly.

The aggravation of having to watch the fuel needle magnifies when our pickups are towing a big trailer or the bed is loaded down with tools and equipment.

Let’s face it: Today’s factory fuel tanks are just too small for those who work their trucks long and hard.

Siphon out as much fuel as possible before removing the stock tank. Also use a good transmission jack and at least one other person in the removal/installation.

Our 9,000-pound ‘08 Mobil Delvac GMC Sierra Crew Cab 4×4 Dually is a prime example of a work truck needing a bigger fuel tank. The Duramax puts a hurt on the 34-gallon factory tank when rolling down the road.

Hitch a 5-ton equipment trailer to the tail and fuel economy drops from 11mpg to around 8mpg, or a driving range of about 240 miles. That’s neither convenient nor acceptable for some diesel pickup owners.

A ready solution is to swap out the plastic OE fuel tank for a higher capacity, steel replacement.

The 56-gallon one we slid under Big Red is a new offering from Transfer Flow (, a long-time builder of premium aftermarket fuel tanks.

Transfer Flow’s OE steel replacement tank is baffled, powder-coated for durability, carries a three-year/60,000-mile warranty, and comes with all the hardware for a clean, factory-look installation. It also meets all federal and state emissions regulations.

It takes special removal tools to disconnect the quick-disconnect fuel lines from the tank’s sender/pump unit. The lines don’t come off without them.

Upgrading to a bigger fuel tank is also the perfect time to upgrade the fuel flow and filter system because some of the same labor is required for both installations.

For that we turned to FASS Fuel Systems ( FASS’s filter system removes all types of fuel contaminants including emulsified water, air/vapor and debris. Much more so than OE water/fuel filters.

The Titanium Series kit we installed also takes a load off the OE fuel pump by using a heavy-duty fuel transfer pump with a flow rate of 95gph for the version used on our 2008 Duramax.

Transfer Flow’s replacement fuel tank has 40 percent more capacity than the stock one under later-model GM diesel Duallies.

FASS claims that’s why their fuel air separation system (F.A.S.S.) improves fuel economy by up to 11 percent in diesel pickups pulling heavy loads and gains of 2-3mpg running empty.

They also say injector life is increased while lowering maintenance costs over the long run.

Truck Supply and Outfitters (, near our offices in Tuscaloosa, Alabama, swapped in the Transfer Flow fuel tank and installed the FASS filter system in our Dually in about a half-day.

Now our Mobil Delvac Big Red is ready for the long run – both literally and ­figuratively.


(To see more step-by-step photos of this installation, go to and click on the “Tech/How-To” tab.)







All of the factory fuel tank hoses and components are re-used in the Transfer Flow tank except the float arm. A resistance (OHM) test is run on the new arm to ensure it’s adjusted accurately.



Transfer Flow:; 800-442-0056

FASS Fuel Systems:; 866-769-3747

Truck Supply and Outfitters:; 205-553-4203