Modern 4WD’s are absolutely incredible when it comes to fuel economy. You can get a full size 4WD that's far more economical than a sedan from 15 years ago, despite the huge weight and size difference. However, there is no beating physics. No matter what you do, here are 10 things that will totally destroy your fuel economy.
Fitting bigger tyres to your 4WD requires more energy to get them rotating, and to keep them rotating. It also messes with the gearing specifically engineered from the factory. Your RPM will drop at any given speed, but your fuel consumption goes up. A minor change in size might not affect your economy much, but increase them by more than 30mm and you’’ll notice a change for sure. It also appears that the smaller the motor in your 4WD, the more a tyre size increase affects your economy.
If you go 100mm bigger, expect to use a LOT more fuel than normal. Remember though, when Working out fuel economy that your odometer and speedometer wont be right – you will have travelled proportionally further than what it says you have. Use a GPS to see how much your speedo and odometer is actually out.
Extra wind resistance
The more air you have to move out of the way, the more fuel you will use. Roof racks, big canopies, light bars up high, roof top tents, towing mirrors, what ever it is, if it sticks out more than the vehicle did originally then you are going to use more fuel.
Interestingly, I’ve heard from several caravan owners who have big vans that tower over the top of their 4WD, that by adding a small boat on the roof racks fuel economy improves substantially. I reckon that’s a major win; take your boat with you and get better fuel economy!
On our Dmax, the Bull Motor body canopy sits about 200mm taller than the roof, and I know it hurts the fuel economy, badly. To make things worse, it also sits proud of the vehicle in width.
It doesn’t matter what vehicle you are driving, if you are going into opposing winds, you are going to use a LOT more fuel. I used to find it very intriguing watching the Exhaust Gas Temperature gauge on our 80 Series Land Cruiser. At 110km/h it would usually sit around 300 – 330 degrees. However, every now and again (often near the coast) it would stay around 380 – 400 degrees.
The reason is really simple; the motor has to work substantially harder to move your vehicle through head winds. These days, you can just look at the instant fuel economy data shown on your 4WD’s dash. I know our Isuzu Dmax in a head wind on the computer will go from 13L/100km to about 20 – 22L/100km.
The only way to reduce this is to slow down, but you are still going to use proportionally more fuel.
If you want to push your 4WD’s cooling system, soft sand is the place to do so. It has to work extremely hard to move the weight of your 4WD through along. Our Land Cruiser used to average anywhere from 15 – 18L/100km. However, on one trip down to Yeagarup through the dunes and along the beach, I emptied my sub tank in under 150km. That equates to 36 litres per 100 kilometres!
By lowering your tyre pressures you will make it slightly easier for your engine, but there’s no beating physics on this one.
Ever wondered why so many people towing caravans sit on 80 – 90km/h? The reason is simple; its to conserve fuel (and its safer to react if something goes wrong). When you tow something heavy, your motor has to work a lot harder and will use a heap more fuel.
The weight of what you are towing is one thing, but the wind resistance is the other – big caravans will use a lot more fuel than ones level with your 4WD. I spoke to a bloke the other day with a Toyota Tundra, who tells me he is averaging 33 litres per 100km towing a 4 tonne van at 100km/h. Without the van its well under 20L/100km.
A lot of 4WD’s will use about ½ more fuel than their original consumption when towing. In WA, you are capped at 100km/h when towing, and this helps to keep the economy in check, but you will consume more energy to move more weight.
Sitting at higher speeds
When we are travelling, occasionally I’ll get tired of overtaking, and will sit behind someone for a long time at a lower speed. After doing this, I’m always impressed at the fuel economy. 80km an hour is probably the ideal fuel saving speed, but it upsets everyone else on the road. 90 - 95 is a good compromise and will save you a fortune. If you aren't towing, test the difference between 100 and 110km/h. I bet it's easily 2L/100km, and that adds up real fast.
The faster you go, the more you pay in fuel. You’ll have to test it out to see what you want to do – sacrifice time for money, or money for time! Just don't sacrifice safety for time.
More aggressive tyre tread
If you change from a set of highway terrain tyres to muddies, not only will you notice a big difference in noise, but you’ll notice a change in your economy. Air doesn’t flow very well through the big chunks of rubber, and as a result you will use more fuel. A lot of money goes into engineering tyres, and a big portion of that is for optimum fuel economy.
The more aggressive your tyres, the more fuel you will use.
Wrong tyre pressures
On the inside of your door lies a little sticker that tells you what tyre pressures you should be running. If you change your tyre size, this will vary, but tyres that are too low in pressure will cause your fuel economy to rocket. You can also go too high, which will give you great economy and wear your tyres out much faster (and not be very safe!).
Its a common myth that the lower your RPM, the better the fuel economy is going to be. People often think that as bigger tyres drop your RPM down you will get better economy, but it couldn't be further from the truth.
Every motor has a sweet spot – a small portion of the RPM range where it generates the most torque, and this is normally where its best fuel economy is. For some vehicles, that's around 1500 RPM, and others its more like 3200 RPM.
If you are operating either side of this, you will use a lot more fuel. This can be due to bigger tyres, but also to different gear ratios, or just poor gear selection on the drivers part. If your gearing is wrong, you will use more fuel.
Over weight 4WD
If your 4WD needs to go on a diet, you'll use more fuel. These days, its scarily easy to go over your GVM and GCM, and there are a LOT of illegal 4WD's out there. Legal or not, the more weight you have to move, the more fuel you need to use. What does your 4WD weigh?
After travelling for 3 months (check out our 3 month trip summary to the Kimberley and Northern Territory!), and using just over 3 grand in diesel, its made obvious why a few simple decisions can cost you a lot more in fuel. Of course, there's a compromise, and I've done lots of things to our Dmax that have hurt its fuel economy. When I got it, 8-9L/100km was very easy to achieve. These days, if I get under 10 things are going well!
Now, you have to decide what's important - modifications, where and how you drive or fuel economy. You can't have a decked out 4WD and the best possible fuel economy; its one or the other.
How's your fuel economy? Do you regret making some changes to your 4WD?