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.

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