Fuel Tank Sending Units and Gauges
What a Fuel Tank Sending Unit Does
A sending unit is the part inside a fuel tank that measures how much fuel is left. A float rides on the surface of the fuel, rises and falls with the level, and sends that reading to a gauge you can see. Without one, the only way to know what is in an auxiliary or transfer tank is to open the cap and look.
On a truck bed fuel tank, the sending unit is installed in the tank and paired with a small gauge mounted in the cab, so you can watch the bed tank the same way you watch your factory fuel gauge.
Two Ways to Read the Fuel Level
There are two common setups, and they solve different problems:
- Sender with in-cab gauge. The sending unit goes in the tank and the gauge mounts on the dash. You read the level while driving, without leaving your seat. This is the setup most people want on an auxiliary tank.
- Tank mounted mechanical gauge. A mechanical fluid level gauge reads directly on the tank itself. No wiring, nothing in the cab, but you have to walk back to the bed to read it. Simple and dependable for tanks that get filled and emptied on a job site rather than on the road.
Sending Units Only Read Accurately on Rectangle Tanks
This is the single most important thing to know before ordering a gauge, and most listings do not mention it.
A float based sending unit assumes the tank holds about the same amount of fuel in every inch of its height. That is true of a rectangle tank. It is not true of a wedge or an L-shape, where the body is much wider at the bottom than at the top. On those shapes the float drops slowly at first and then quickly, so the gauge reads wrong through most of the tank. A gauge that says half when the tank is nearly full is worse than no gauge at all.
For that reason, the sender and in-cab gauge option is available on rectangle tanks only:
| Tank Body Style | Sender and Gauge |
|---|---|
| Rectangle, any capacity | Yes |
| Rectangle with toolbox combo | Yes |
| Split rectangle (two compartments) | Yes, one gauge per compartment |
| L-shape | No, float cannot read the shape |
| Wedge | No, float cannot read the shape |
Split tanks are worth a note of their own. A split is built as two separate tanks welded together, with its own fill point on each side, so each compartment needs its own sending unit and its own gauge if you want to monitor both.
Ordering a Sender and Gauge With Your Tank
The sending unit is installed at the factory when your tank is built, not added afterward in your driveway. On our rectangle auxiliary fuel tanks you can add the sender and dash mount gauge as an option when you order, and the tank ships with it already fitted.
Because it is a built to order item, adding a gauge adds roughly a week to the normal ship time. Tanks without one usually ship within a few business days.
Sending Units for Boats and RVs
Marine and RV fuel senders work on the same float principle, but they are sized and calibrated to the specific tank they sit in, which is why they are almost always sourced with the tank rather than bought separately. We do not sell standalone replacement senders. If you are building or replacing a tank in a boat or camper, start with the tank itself: see our RV and marine tanks.
Common Questions
Can I add a sending unit to a tank I already own?
Not through us. The sender is fitted when the tank is manufactured, so it needs to be ordered with the tank.
Why can't I get a gauge on a wedge or L-shape tank?
The float inside a sending unit tracks height, not volume. On a wedge or L-shape the bottom of the tank holds far more fuel than the top, so the reading would be inaccurate through most of the tank. The manufacturer does not recommend it, and neither do we.
Does a split tank need two gauges?
Yes, if you want to monitor both sides. A split tank is two welded tanks sharing one body, so each compartment reads separately.
Does the gauge run off the truck's power?
The in-cab gauge is wired into the truck like any other accessory gauge. The tank mounted mechanical option needs no power at all.
Will adding a gauge delay my order?
Slightly. Most tanks ship within a few business days, and adding a gauge or powder coat adds about a week.
Find a Tank You Can Keep an Eye On
Browse rectangle diesel auxiliary tanks and gasoline auxiliary tanks, or see all truck bed transfer tanks. New to auxiliary tanks? Start with how gravity feed fuel tanks work. Questions about which setup fits your truck? Call us at (863) 261-8388.