Adventioneering

When selecting a vehicle for vehicle dependent expedition, an excellent criterion to look at is how efficient it is. On an expedition, the vehicle with the lowest ltr/100km rating is not necessarily the most efficient. To look at the whole picture, you must look at the fuel-econ per useful payload capacity.

A larger truck may have a very poor fuel-econ rating compared to a passenger car, or light truck, but it can carry far more payload – so laden it is actually using less fuel per kg and is therefore a more efficient vehicle for moving weight over distance.

Given a vehicle, there are several techniques for extending it’s range, most of which come down to figuring out how to transport more fuel. The more fuel you can transport, the more range you have. The rated weight never-to-exceed of the vehicle (GVWR) is usually the limiting factor.

The overall process can basically be stated as: estimate the expected fuel-econ of the vehicle, figure out how much weight you have available for fuel, and figure out how much range that buys you. Correct en-route as required.

On a stock Land Rover Discovery I, the GVWR is 2730kg. The wet, empty rate of the vehicle is 2020kg yielding 710kg for passengers and equipment. Given two passengers of about 75kg each this now leaves us with 560kg of remaining weight.

The mixed-use fuel-econ of this vehicle in good tune is around 15.7l/100km. Fuel-econ will be considerably better or worse in reality depending on a) speed b) terrain c) start and stops, which is why it is crucial to monitor fuel consumption en-route.

Weight (including of course weight brought on by carrying additional fuel) can be a major or minor factor in the fuel econ of the vehicle depending on the user. If you must climb through a mountain range, for example, weight will be a major factor. If you are driving a fixed speed for long distances over flat terrain, it will be almost no factor at all.

The reason for this is that once a weight is accelerated the only thrust that must be provided is to overcome parasitic losses; air drag (major), friction losses at the wheel interface (minor), and losses in the driveline (tiny).

A vehicle has the same drag coefficient whether it’s loaded or not – as long as the weight is inside the vehicle cabin and not on a roof rack. When climbing a grade, however, more weight must be lifted so more work must be done, resulting in greater fuel consumption for a laden vehicle than un-laden.

An empty 20ltr jerry can w/o spout weighs 4.3kg. 1 liter of petrol is about .73722kg/ltr @ 25°C for a total unit weight of ~ 19.05kg/20ltr. This yields 580 liters of gas beyond the 89 liters that fits in the stock tank.

Assuming there was no useful cargo, food, or other supplies – nothing but fuel – the vehicle could support 29 jerry cans (assuming you could find the room!) yielding 580 liters of petrol. This is enough fuel at the stated fuel-econ for 3694km.

This is obviously an extreme, unpractical example – there is probably little point in making such an endurance journey if you can’t take anything with you – but it illustrates the overall process.

In the real world, your vehicle is unlikely to have the curb-weight stated in it’s manual, not after you add off-road tires, a winch and associated bumper, recovery hardware, and possibly a roof rack. It’s crucial that you weigh your vehicle – fully wet. This is an easy process to do, as there are commercial scales available at almost every truck stop in North America. If a government weigh station looks not very busy, they will also sometimes weigh a car or truck. Also in some areas (notably Colorado) the scales are automatic, so you can drive on when the station is closed and get your weight even though no one is there.

If the terrain is forgiving, an excellent way to increase range is to take a fuel trailer, which have several benefits. The major advantage is that a trailer does not deduct more than its tongue weight from the vehicle’s GVWR. You can store additional gear on the trailer, and (provided you can secure it) you can leave it at a base-camp location and then make daytrips away from it.

Depending on the requirements of the expedition, another option is to bring a larger vehicle with a much more efficient GVWR (Unimog, Pinzgauer, half-deuce, etc). It can supply base-camp where the smaller vehicles can refuel.

This article is part of the Vehicle Dependent Expedition section.