How to Install a Spare Tire Water Tank

One of the best things about the Mercedes Sprinter for van conversions is the spare tire carrier under the rear of the van. Remove the spare tire and you've got a ready-made mounting point for a freshwater tank — no fabrication required, no interior space sacrificed, and the weight sits low on the chassis where it belongs.

We install spare tire water tanks on most of our Sprinter builds at our San Diego shop. It's one of the most practical upgrades for any Sprinter conversion, and the install is straightforward once you know the process. This guide covers everything: tank selection, mounting, plumbing, fill setup, and the mistakes we've learned to avoid.

Which Spare Tire Water Tank Should I Buy?

Several manufacturers make tanks specifically designed for the Sprinter spare tire location. The main variables are capacity, material, and how they mount.

• Class B Components (21 gallon): Purpose-built for the Sprinter spare tire carrier. Polyethylene, food-grade, comes with a dedicated mounting bracket. This is the most popular option and the one we install most often. Around $450–$550 with bracket.
• Aluminess spare tire tank: Aluminum construction, lightweight and durable. Higher price point ($600–$700) but won't crack in freezing conditions the way plastic can. Good choice if you spend time in cold climates.
• Custom fabricated tanks: Some shops fabricate stainless or aluminum tanks to exact specifications. More expensive but lets you maximize capacity for your specific setup. Typically $500–$700+ depending on size and material.

Capacity ranges from about 15–25 gallons depending on the model. For a solo traveler, a spare tire tank alone may be enough freshwater. For a couple, it works well as either your primary tank or a supplemental tank paired with an interior tank for additional capacity.

How Do I Install a Spare Tire Water Tank Step by Step?

Step 1: Remove the Spare Tire

Lower the spare using the factory winch mechanism. On a Sprinter, the access point is typically a hex bolt behind a cover near the rear bumper. Lower the tire completely, unhook the cable, and set the tire aside. Keep it — you may want it for road trips where you'd rather have a spare than extra water.

Step 2: Inspect and Prep the Carrier

Clean the carrier area thoroughly. Check for rust, especially around the bolt holes and the cable mechanism. Wire brush any surface rust and hit it with rust-inhibiting primer. This area is exposed to road spray and salt, so any corrosion will only get worse if you don't address it now.

Step 3: Test-Fit the Tank

Before committing, hold the tank in position and check clearance on all sides. Verify that the tank doesn't contact the exhaust pipe, driveshaft, or any other undercarriage components. Check ground clearance — get under the van and measure from the lowest point of the tank to the ground.

Step 4: Install the Mounting Bracket

Bolt the tank bracket to the spare tire carrier mounting points. Use stainless hardware — the factory bolts are fine structurally but will corrode over time in an exposed location. Apply thread locker to the bolts. If your tank doesn't include a purpose-built bracket, a universal mounting plate with strap holds works well.

Place rubber isolation pads between the bracket and the tank. Metal-on-plastic contact points will wear through the tank wall over thousands of miles of road vibration.

Step 5: Mount the Tank

Secure the tank to the bracket. If using straps, tighten them evenly and check that the tank has zero movement when you push on it from any direction. A tank that can shift even slightly will wear against the bracket and eventually develop a leak or crack.

Step 6: Set Up the Fill Port and Vent

You have two options for filling:

• Direct tank fill: Install a fill port directly on the tank (if not pre-installed). You'll fill from under the van by connecting a hose to the port. Simple but less convenient.
• Remote fill: Run a fill hose from the tank to an exterior fill port mounted in the body of the van — usually behind a small access door on the outside. More work to install but much more convenient for daily filling.

Either way, the tank needs a vent. When water goes in, air needs to come out. Install a vent fitting on top of the tank and route the vent line upward (above the maximum water level) to prevent siphoning. A small screened vent cap keeps bugs and debris out.

Step 7: Run Plumbing into the Van

Drill a hole through the van floor above the tank outlet location. Use a step bit for a clean hole. Install a bulkhead fitting or rubber grommet, and seal around it with butyl tape or Dicor. Water getting into the floor sandwich (metal, insulation, subfloor) is a problem you don't want.

Route your supply line from the tank outlet, through the floor, and to your water pump. Use PEX or braided hose — both handle vibration well. Avoid rigid fittings that could crack from road vibration.

Step 8: Connect and Test

Connect the supply line to the inlet side of your 12V water pump. From the pump, run lines to all your fixtures — sink, shower, and outdoor shower if you have one.

Fill the tank completely and inspect every fitting and connection for leaks. Then run the pump and check again under pressure. Finally, take the van for a drive over bumps and rough road, come back, and check everything one more time. Road vibration reveals leaks that bench testing misses.

What Mistakes Should I Avoid with a Spare Tire Water Tank?

• Skipping rubber isolation: Metal brackets directly against a plastic tank will wear a hole in the tank wall. Always use rubber pads at every contact point.
• Forgetting the vent: Without a vent, the tank creates a vacuum as water is drawn out, slowing flow to a trickle. And without a vent, filling is painfully slow because trapped air has nowhere to go.
• Rigid fittings on a moving vehicle: Use flexible connections (PEX, braided hose) between the tank and the interior plumbing. Rigid pipe and fittings will vibrate loose or crack over time.
• Not sealing the floor penetration: The hole where your plumbing passes through the floor is a leak path for road spray and water. Seal it properly with a bulkhead fitting and sealant.
• Ignoring the drain: Install a low-point drain valve so you can empty the tank completely for winterizing or maintenance. A tank you can't fully drain is a tank you can't properly sanitize.
• Not keeping the spare tire: If you get a flat on a remote road, you'll wish you had the spare. Keep it in the van's garage area or on a rear-mounted carrier for long trips.

Is a Spare Tire Water Tank Right for My Sprinter Build?

A spare tire water tank makes sense if:

• You have a Sprinter (any generation) and want freshwater without giving up interior space
• You need more water capacity and have already used your interior tank space
• You want the weight low and centered for better handling
• You're doing a new build and want to maximize interior layout flexibility

This is also a popular retrofit for existing builds. If your van is already finished and you need more water capacity, adding a spare tire tank doesn't require tearing into the interior — just underslung mounting, one floor penetration, and a connection to your existing plumbing.

For more on freshwater system design, see our freshwater systems guide. For the full plumbing overview, we cover tanks, pumps, water heating, and grey water handling.