September 20, 2026 Colby Hill

Fuel Range Offroad: How Much to Carry, and Where to Put It

Watch a staging area on a Friday night and you will see the same setup over and over: two five-gallon cans strapped to the roof rack, up where the heaviest, most flammable thing on the truck has the worst possible effect on how the rig drives. Underneath that is usually a second mistake — nobody worked out whether they needed ten gallons in the first place. They bought cans because cans look like preparation.

Extra fuel is worth carrying on plenty of trips. But it is the one piece of gear where guessing costs you twice: once in payload and center of gravity you did not need to spend, and once when a badly mounted can leaks into the bed on a hot day. Work out the number first, then pick and mount the hardware.

Start with your real range, not the window sticker

Your highway mileage is irrelevant to this calculation. The number that matters is what the truck burns on the kind of driving the trip actually contains, and that number is worse than most people expect.

Low range is expensive

Crawling in low range at walking speed, the engine turns a lot of revolutions to cover very little ground. Add a torque converter slipping, larger and heavier tires, and a loaded rack pushing air, and it is normal to see consumption double or worse against a highway cruise. Deep sand at low pressure with the throttle held open is the most fuel-hungry thing most rigs will ever do.

Idling counts too. A long recovery, a lunch stop with the A/C running, twenty minutes of winching — none of that moves you toward the next pump, and all of it burns.

Do the math with a real reserve

Work it in three steps:

Measure the longest fuel-to-fuel leg of the route, including the drive to the trailhead and the drive out. Not the trail mileage — the whole leg between two pumps you are confident will be open and have fuel.
Estimate consumption in two parts: highway miles at your observed loaded highway mileage, and trail miles at a conservative fraction of it. Halving your highway number for trail miles is a reasonable starting assumption for most rigs; a third is more honest for sand or heavy crawling.
Plan to arrive with a quarter tank, not a splash. That margin covers a missed turn, a closed station, a detour around a washout, or the day the route takes six hours longer than planned.

Run that and you often find the gap is four gallons, not ten. One can, carried well, beats two carried badly. Work this out at the same time you plan the route — the fuel stops are part of the route, and if you are building the trip properly you already have them marked. Our guide to off-road navigation and route planning covers the rest of that process.

There is a second reason for that floor. Fuel gauges are not linear, and a tank reading a quarter full on level ground can uncover the pickup on a long side slope or a steep climb. Keeping fuel over the pickup matters as much as keeping it in the tank.

What a fuel can actually has to do

A can that lives in a garage and gets used twice a summer has an easy life. A can strapped to a vehicle spends days vibrating, heat-cycling from cold morning to hot afternoon, and gaining and losing altitude. That is a much harder job, and it is where cheap cans fail.

Steel military-pattern cans

The NATO-style steel can is the standard for a reason. It seals on a gasket with a cam lever, has a proper internal breather tube so it pours smoothly, and the three-handle design lets two people pass one or one person carry two. It is also the only common type that reliably survives being mounted outside a vehicle for years.

The costs are real: it is noticeably heavier empty than plastic, and it will rust if the coating gets chipped and stays wet. Buy the can and the spout together — the spout is a separate item on most of these and a mismatched one leaks.

Plastic cans

Plastic is lighter, cheaper, and will not rust. It also flexes with temperature and pressure, which is what you feel when a can that was flat this morning is drum-tight by afternoon. Thin consumer cans mounted outside on a rough trail wear through where they contact the mount, and their cap and vent designs are not built for constant vibration. If you go plastic, use a heavy-wall can intended for vehicle mounting and inspect the contact points every trip.

The spout problem

Modern consumer spouts are designed to eliminate vapor escape: no separate vent hole, and an automatic shutoff that engages against the filler neck. They are awful to pour with, and the spill they cause is its own hazard — fuel on a hot exhaust or brake rotor is not a theoretical problem. Buy a can whose pour system is built for the job, and keep a funnel with it.

Where the weight goes

Gasoline runs roughly six pounds a gallon and diesel about seven, so a filled five-gallon steel can is in the neighborhood of forty pounds with the can itself. Two of them is the weight of a passenger, and unlike a passenger it sits wherever you bolt it.

Ranked from best to worst:

Rear bumper or swing-out carrier. Low, outside the cab, easy to reach, and it keeps fuel vapor away from where you sleep. It adds rear overhang weight, which affects departure angle and rear spring load, but it is the cleanest answer for most rigs.
In the bed, against the bulkhead. Low and forward of the rear axle. Excellent if your bed setup gives the can a flat, ventilated home where it cannot slide or chafe.
Roof rack. The worst of the three. Forty pounds at roof height does more to your center of gravity than any other single item you carry, and you have to lift it down over your head to use it. If you must, put it at the very rear of the rack and read our piece on roof racks and payload first — the limits up there are lower than people assume.
Inside the cab or a closed cargo area. Don't. Even a good can vents, and fuel vapor in an enclosed space you breathe in is the one mistake with no upside.

Whatever you choose comes out of the same payload budget as water, recovery gear, and passengers. If you are already near the number, adding fuel means removing something else — our guide to on-board water storage covers the same math for the other liquid you carry.

Mounting it so it stays put

A forty-pound mass on a vehicle shaken for eight hours finds every weakness in how you attached it.

Use a real mount, not straps alone. A proper can holder captures the can on multiple faces so it cannot rock. Ratchet straps are a great supplement and a poor primary — tie-downs stretch, loosen, and abrade.
Through-bolt with backing plates. Sheet metal and thin rack tubing will tear out. Spread the load with a plate or a purpose-built bracket rated for the weight, and use locking hardware — nylon inserts or thread locker, because vibration will back off a plain nut.
Isolate the contact points. Bare metal on bare metal wears a hole. Rubber or UHMW pads between can and mount stop the chafing and quiet the rattle.
Mount it upright with the cap accessible. A can you have to unbolt to use will not get used, and one lying on its side eventually weeps at the gasket.
Re-torque after the first trip. Everything settles. Add it to your post-trip walkaround.

Venting, heat, and pressure

A sealed can of fuel is a pressure vessel that follows the weather. Fill it cool at a low-elevation station, drive to a high trailhead on a hot afternoon, and the contents expand while outside pressure drops. Plastic cans bulge visibly; steel cans build pressure quietly. Three habits handle it:

Never fill to the absolute top. Leave the expansion space the can was designed with — the fill line is there for a reason.
Burp the can when you notice it swelling, in the shade, away from ignition sources, with the cap pointed away from your face. Crack it slowly rather than flipping the lever open.
Open a hot can with the same caution you would use on a hot radiator cap. It can spit.

Filling is its own hazard. Static is the reason station signs tell you to put the can on the ground, and it is not superstition — a can on a plastic bedliner is insulated, and the charge that builds while fuel flows has nowhere to go but a spark at the nozzle. Set it on the pavement, keep the nozzle touching the neck, and fill slowly.

Fuel does not keep forever

The gallon you carry for emergencies is only useful if it still burns. Pump gasoline blended with ethanol starts degrading within a few months and pulls moisture out of the air; enough water separates out as a layer the engine cannot run on. Heat and air accelerate all of it, and a can strapped outside in summer gets plenty of both.

Treat carried fuel with stabilizer if it is going to sit more than a month or two, and label the can with the fill date.
Rotate it. Pour it into the tank at the end of a trip and refill fresh rather than letting the same five gallons live out there for a season.
Keep the can full rather than half full when it is stored. Less air in the can means less moisture and less oxidation.
Diesel has different problems, not fewer: it gels in cold weather and it grows microbial contamination when water gets in. Winter carriers need a treated fuel and a plan for the cold.

If you ever need to move fuel the other direction — out of the tank, into a stranded friend's rig — carry a proper jiggler-style siphon hose. Modern anti-siphon screens defeat most improvised attempts anyway, and nobody should be starting a siphon with their mouth.

When cans stop making sense

Past ten gallons or so, handling the cans becomes the problem. Each is a forty-pound lift, a spill opportunity, and something to store when empty. If you consistently need serious extra range, a larger or auxiliary tank plumbed into the vehicle is the better answer: the fuel rides low, you never touch it, and there is no pouring at the side of the trail.

The tradeoffs are cost, install complexity, and permanent weight whether the trip needs it or not. In-bed transfer tanks are largely a diesel solution; the rules for transporting and transferring gasoline that way are stricter, so check what a given tank is certified for before assuming it works for your fuel. And check clearance — a larger replacement tank often hangs lower than the original.

For most people the honest answer is one good can and a better plan. Five gallons you probably will not need is cheap insurance. Fifteen because it looks serious is a handling penalty you pay on every mile.

The short version

Calculate the longest fuel-to-fuel leg first, use a conservative trail mileage number, and plan to arrive with a quarter tank.
Most trips need one can, not two. Buy the range you actually need.
Steel military-pattern cans survive vehicle mounting; thin consumer plastic cans usually do not.
Rear carrier or bed, low and outside the cab. Never the interior, and the roof only as a last resort.
Through-bolt a real mount with backing plates and isolate the contact points. Straps supplement, they do not hold.
Leave expansion room, burp it in the shade, and fill on the ground to avoid static.
Stabilize, date, and rotate. Old fuel is not a reserve.

Get the range math right and the hardware choice gets easy. If you are putting the carrying side together — mounts, bed storage, rack hardware, and the rest of the camp kit — start with our overland gear collection and the storage and organization and camping accessories pages.