TorqueTalk

With the cost of fuel having been in the news lately and the projection that the cost per litre is likely to be around $ 3.00 by Christmas, we start to look at the fuel economy of our vehicles. I looked at my fuel consumption after a recent trip away and I used to get significantly better when I first purchased the vehicle. How come?

I just topped up the fuel tank of the Discovery taking only 60 litres and paying 2.29 per litre. Given the seemingly sudden rise in the price of fuel, especially the premium fuel, that is starting to hurt, especially when using it regularly. I had filled up after returning from the Hawke's Bay Winch Challenge and the distance travelled meant that I was returning 18 litres per 100kms. That was a bit up on the trip down which had returned 16 litres per 100kms over similar distance.

So why the sudden drop in economy, what had changed? Was that because it is mostly downhill to Hawke's Bay from Auckland and therefore uphill coming back? Surely not.

This got me thinking as it happens to us all when we suddenly realise that we just aren’t getting the same economy that we did previously. Is there something paranormal happening or is there a logical explanation?

Actually it’s all very logical when you think about it.

That shiny, straight and standard vehicle that you drove around on standard rubber for a few weeks is very different to the one you have now. To start with it had a sleek shiny body which manufacturers had spent many hours in wind tunnel testing and developing a reduced wind resistance and it returned good fuel economy.

While there are many aspects attributing to poor fuel consumption let’s look at some of the most obvious ones.

Servicing; Have you ever noticed how the vehicle feels better, livelier after a good service? Yes, regular servicing with fresh clean oils helps keep the engine and drive train in good smooth running order and reduces friction.

Weight; Weight is a great user of fuel, especially on the winding and hilly road through NZ. We tend to add significant weight to our vehicles in the form of recovery gear, tools, spare water and fluids, bullbars, winches, side bars, etc. All this increases the overall weight and means engine has to work harder to propel the vehicle.

Wind resistance; A factor more so at cruising speeds. We change the manufacturers clean sleek lines of the vehicle by fitting accessories that make the vehicle less aerodynamic, such as bullbars, roof racks and driving lights. Even the modern light bar can have an influence on wind resistance and ultimately fuel usage.

Tyre size; This is probably one that is a big factor to poor economy and loss of power as we fit larger aggressive tyres, often on heavier steel rims. The larger rotating diameter takes more engine torque to turn the wheel and tread blocks on mud tyres significantly add to the rolling resistance.

To combat all these factors we put the foot down a bit further and that’s where your fuel economy has gone.

Looking at my situation, and all of the above is true. I have removed the original front spoiler and lif ted the vehicle so it is no longer as wind resistant as it was. Fitting a bullbar and the Warn 8274 winch, with 50 metres of wire rope, added over 100kgs extra in weight. The full recovery kit of shackles, ropes, tree protectors, snatch blocks and spade plus the extra water and oil I carry, tools, tyre compressor along with other ‘ in case’ items like jumper leads, I really have added to the overall weight.

Of course I have the BFG mud tyres which are also significantly taller and increase the gearing of the vehicle that it requires more torque and fuel to get it rolling.

But all these were applicable on the trip down I here you say? Yes but one overriding factor was I took the back roads rather than state highway on the return trip. While distance was about same, it involved more hills and winding roads. Even so I was surprised at how much more fuel it used and how that translated back to the now empty wallet.

With the trip being 1100kms return the extra two litres per 100 equates to roughly an extra $ 50.00 in costs at current pricing. Time to go on a diet.