In this instalment of our ultimate guide to towing series, Robert Pepper discusses how to save fuel when towing your caravan or trailer.
Fuel costs are becoming a bigger and bigger part of caravanning, and there’s no going back to the old prices. Let’s look at a weekend away. If fuel costs $2.20 a litre, and you drive 1,500 kilometres at 15L/100km, then you’ll spend nearly $500 on fuel. That’s a significant extra cost on top of everything else.
If we look at a longer trip, say 7,000km, then that’s more like $2,300. And if we change the number to 18L/100km, still at $2.20, then we spend $600 in fuel for our weekend away, and $2,700 for the longer trip.
We’d all like to get that number down, and given we can’t do much about the price of fuel, we need to look at how to get the most kilometers from every one of those expensive litres. Now I’ll warn you; there is no quick-fix, no magic or silver bullet, but I can offer some useful tips and tell you what doesn’t work.
What works to reduce fuel consumption?
Speed
You didn’t want to hear this, but the single biggest way to reduce fuel consumption is to reduce speed. You don’t really think of air as a gas, but it is, and we move through it all the time, like wading through water, except we can’t (usually) see air. Just like wading through water, moving through the air takes effort; try walking into a headwind for example. The effort required to move through the air is called “drag”, and if you reduce drag, you can reduce the energy required to move your rig and therefore, go further for every litre of fuel you buy.
At very low speeds, like walking pace, there’s almost no drag, and the car’s energy use is mostly overcoming friction in the engine and transmission, rolling the tyres and so on. But once you get a car above about 40km/h the major energy use is overcoming drag, and that becomes even more important at cruise speeds like 80 to 110km/h.
Have you heard of “speed squared law”?
Now here is the speed trick. There is something called the “speed squared law”, which works in your favour. Imagine a car travelling at 20km/h. You double speed to 40km/h. So that’s double the drag, right? No, it’s the square of the increase, so 2 x 2 = 4, four times the drag. Now take that car from 20km/h to 60km/h, three times the speed. That would be 3 x 3 = 9, or NINE times the drag! In other words, drag builds up very, very quickly.
So to recap. The major energy use in cruise is drag. And drag builds up quickly. So going from 100 to 110km/h might be a 10% increase in speed, but it’ll more like a 20% increase in drag, and something close to that in fuel consumption. You can test this easily enough on any freeway; cruise at say 90km/h, reset your fuel consumption indicator, drive for a few km, then increase speed to 100 and try again.
The sweet spot for cruising range and fuel efficiency is likely to be around 60-80km/h, very much dependent on the vehicle, drag profile and many other factors. Of course, you can’t always cruise that slowly or would want to, but the point is that freeway cruising speeds are definitely not ideal for fuel efficiency and range. And drag is a constant once you’re above suburban speeds, whether you’re on the flat, downhill or uphill.
Weight
“Make it lighter!” is what you’ll often hear people say to reduce fuel consumption, and they’re right, to a point. That point being uphill or accelerating, because that’s really the only time weight makes a difference. Once you’re in cruise on the flat the weight of the vehicle doesn’t make much difference at all, it’s more about drag.
Wheels, tyres and tyre pressures
Tyres soak up a lot of energy, which is why EVs run special low-rolling-resistance tyres. However, most towers will choose tyres that increase fuel consumption, for example all-terrain treads, light-truck construction, and taller and wider diameters. Now of course there are good reasons for choosing such tyres and I have, and will advocate for such changes. But be aware there are downsides and fuel efficiency is the main one.
A good way to compromise is this; don’t go for wide tyres, there’s no performance advantage. Keep your diameter increase small, under 50mm which is also typically the legal limit. Fit high-profile tyres as they’re lighter and wheels/tyres are rotating mass which requires around 1.6 times more energy to rotate; this means 16” rims not 20”. Don’t go for mud tyres unless you really need them.
Finally, keep tyre pressures up, usually 3-4psi over placard and more on the rear when towing for that towball mass load. Check tyres for uneven wear as always, and get a wheel alignment done as if that’s out, you’ll damage expensive tyres and use more fuel.
Servicing
A car with fresh oil, belts, filters and so on will use less fuel than one with worn components.
Driving technique
Aside from speed, this is the best way to improve fuel efficiency. We’ve covered speed and the effect that has, and the second biggest use of fuel is acceleration. So, to drive for efficiency you keep your top speed down, and avoid acceleration, or to consider it another way, avoid slowing down as every time you slow down you’ll need to accelerate again.
The way you avoid accelerating is observation and vision; reading the road so you don’t actually come to a stop. For example, not rushing up to a red traffic light if you could avoid stopping. Watching traffic so you can smoothly merge and not need sudden acceleration.
Avoiding braking works too. Rather than keep the power on then brake firmly for a corner, consider easing off the throttle early and letting the car’s natural drag brake the rig as that way you’re spending less time with your foot on the throttle.
Learn some basic cornering techniques so you don’t need to crawl around corners. This isn’t a suggestion to be a racing driver, but those techniques do help as the less you need to slow for a corner, the less you need to accelerate on the other side.
Over the top of crests let the speed fall away and then over the top use the downhill to regain speed. That’s not a bad safety idea too.
However, please be mindful that taken to extremes, fuel efficiency techniques can inconvenience or even worse, endanger other road users so always think safety first and foremost.
Weather
Rain increases consumption as there’s extra drag from the water, especially tyres, your wipers are working, headlights often on and demister working. Then there’s headwinds which can really kill your consumption. Hot weather means the aircon will be on and the engine working with hot, inefficient air.
You will save fuel if you leave early for your distance trip in the cooler morning air, less traffic, and ideally no wind or rain.
Tunes
Tuning the ECU may make a difference, but beware of the many cheap tuners who will make unwarranted compromises, knowingly or unknowingly. Stay clear of any big claims and if there isn’t a detailed explanation of pros and cons, you’re talking to the wrong people.
What doesn’t work?
Cruise Control
Cruise control can save fuel but only if the terrain doesn’t change such as a long, flat, straight road. In any other circumstance you as a human can do better if you follow these tips. For example, cruise control will increase throttle to maintain momentum up a hill right up until the crest, and then back right off. A human would be smarter and allow speed to bleed off uphill, saving fuel.
Fuel-saving fuel additives or other fluids
Don’t bother.
Throttle controllers
All these do is change the throttle sensitivity. No effect on fuel consumption.
Tow electric
Thought about the ultimate energy saving device, an electric towcar? You’ll certainly reduce your energy costs, but there are at the moment still quite a few disadvantages. First, towing dramatically reduces the EV’s already short range compared to diesel, and you’ll need to use expensive fast chargers which probably aren’t trailer-friendly, often requiring an unhitch.
As of April 2024, there are few EV towcars in Australia and none can tow over 2,500kg. But if you can make it work, then aside from range, EV towing is a much better experience than diesel or petrol towing, and the EV towing options will only get better over time so keep it in mind for the future.
