How to Measure Your Work Truck Fuel Door Opening

The single most common question we get is simple: will it fit my truck? The answer is almost always yes, but which size you need depends on the body, not the chassis. A Ford F-550 with a Knapheide service body and a Ford F-550 with a Reading utility body can have different fuel door openings. Three minutes with a tape measure settles it.

What You Need

• A tape measure marked in inches
• Your phone camera
• Two minutes of daylight

Step One: Find the Opening, Not the Door

You are measuring the hole in the body, not the flap that covers it. Open the existing fuel door all the way, or if the body has no door at all and the filler is exposed, measure the cutout in the sheet metal or the recessed housing around the filler neck.

Do not measure the door panel itself. On many upfitted bodies the door overlaps the opening by half an inch or more on each side, and that overlap will send you to the wrong size.

Step Two: Measure Width and Height

Measure the opening edge to edge at its widest point, then top to bottom at its tallest point. Write both numbers down. Round to the nearest eighth of an inch. If the opening is round, measure the diameter at its widest and note that it is round.

Step Three: Measure the Depth

This one gets skipped and it matters. Hold the tape across the face of the opening and measure how far back it is from the outer body surface to the fuel cap. On a flush-mount body this may be an inch or less. On a deeply recessed housing it can be three inches or more. Depth tells us whether you need a finishing bezel to close the gap cleanly.

Step Four: Check What Is Around It

Look at the four inches of flat body surface surrounding the opening. Our door mounts to that surface, so note anything in the way:

• A compartment door hinge or latch that swings into the area
• A body seam, rivet line, or rub rail crossing the mounting surface
• A DEF filler sitting right next to the diesel filler
• A ladder rack upright, mud flap bracket, or step

Step Five: Take Three Photos

Photos catch what measurements miss. Take one straight-on shot of the open fuel door area from about three feet back. Take one at an angle so we can see the depth and how the housing is built. Take one wide shot showing the whole side of the truck so we can see the body style and anything nearby.

Step Six: Send It To Us Before You Order

Text or email the three photos and your three numbers, tell us the chassis year, make, and model plus the body builder if you know it, and we will confirm the exact part number. We would rather spend two minutes confirming than have you spend a week on an exchange.

Common Measuring Mistakes

• Measuring the door instead of the opening. The most frequent error by a wide margin.
• Measuring one truck and ordering for a mixed fleet. If your fleet has three body types, measure one of each.
• Forgetting the depth. Without it we cannot tell you whether a bezel finishes the job cleanly.
• Assuming the model year carries over. Upfitters change tooling. A 2022 and a 2025 of the same body can differ.

If You Are Ordering for a Whole Fleet

Send us your vehicle list with chassis and body types and we will build the size breakdown for you, along with a written quote. Most mixed fleets end up standardizing on two part numbers, which keeps the parts room simple for years afterward.

Protect the Fuel Door, Not Just the Cap

The Work Truck Fuel Vault is a patent-pending armored locking fuel door that bolts over the existing fuel door opening on a commercial work truck. It installs in minutes with common hand tools, needs no drilling into the chassis and no wiring, and it does not touch the fuel system. Two sizes cover the overwhelming majority of work truck openings.

Every unit ships free within the fifty United States and carries a limited lifetime warranty against defects plus a 30-day money-back guarantee. Not sure which size fits your truck? Send us a photo and a tape measure reading and we will confirm it before you order, or call 941-541-0089 and talk to a real person in Sarasota, Florida.