Fuel Economy Facts & Myths: How you really can get more between fill-ups Part 1:
Tuesday, September 16, 2008 at 8:51 AM
Now, more than ever, drivers are looking for ways to improve their cars fuel economy in an effort to reduce fuel consumption and expenditure.
There have been many tips and tricks promoted and popularised over the years, but how many of these actually stand up to the test? Some are just the motoring equivalent of Urban Myths, others may have held true in the past but are no longer relevant with todays technological advances.
In this (and next week's blog) we will review some of these ideas and see which have merit and which can be discarded. In the next article, well be looking at some fuel-saving ideas that actually work. For now though, lets explore a few of the ideas that dont stand up to closer inspection:
Idle or Stop & Start?
They say: "Idling your engine is more efficient than starting up and turning off repeatedly."
We say: This was certainly true back when all cars were equipped with carburettors, and many drivers still believe that repeatedly starting up and turning off your car is a great way to waste fuel.
But with todays modern fuel-injection technology, drivers will actually save fuel by turning off their engine rather than letting the car needlessly idle - just look at the Toyota Prius.
Of course, this doesnt mean that drivers should shut down the engine every time they get stuck in traffic or stop at lights, but if it looks like you might be pulling up for any longer than a minute or so, its worth turning off your car. Naturally, if youre driving an older vehicle this practice may still work for you.
They say: "You should warm up your vehicle in the morning before driving."
We say: Modern engines warm up quicker and are more fuel-efficient when moving on the road, as opposed to warming up while standing still.
A breath of fresh air
They say: "Leaving your windows rolled down creates an aerodynamic drag on your car, cutting down on your fuel efficiency."
They also say: "Running your Air-Conditioning drains energy, and therefore fuel."
We say: Well, both are true to an extent - but not to any great degree. Plus of course, if you turn your air-conditioning off, you may well need to open your windows, so any benefit from one will be cancelled out by the other.
Two separate U.S. studies conducted in 2005 by Consumer Reports and Edmunds.com looked at how running the A/C and opening the windows affected the fuel economy of different vehicles travelling at highway speeds. What they found was that running the air conditioner reduced each vehicle's fuel economy by less than 1 mile per gallon (on the highway).
The Consumer Reports' study found that, while opening the windows does increase the aerodynamic drag on a car, it does not have a measurable effect on the vehicle's fuel economy even at highway speeds. Please note though that opening a sunroof as well as the windows will create some extra drag. So, as far as highway and general driving is concerned, you're not going to save a whole lot of fuel with either option.
However, if you're driving around town on errands, you might save some fuel by rolling down the windows instead of using the A/C if you can stand the pollution (the U.S. Department of Energy suggests that turning off the air conditioner may improve fuel economy when driving at speeds under 40 mph).
Hot & Cold
They say: "Fill up when its cool"
We say: The reasoning behind this one is related to the density of fuel at various temperatures. During warm weather, its supposedly best to refuel in the early morning or late evening, when temperatures have dipped, because the fuel will be denser and (so the theory goes) the denser the fuel, the better your economy.
Modern fuel pumps are temperature-compensated to regulate fuel density, and even if they werent, fuel tank temperatures generally remain stable all day, making any supposed advantage of going out of your way to fill-up at a certain time negligible.
They say: "Park your car in the shade, as heat from the sun will result in fuel evaporation and loss."
We say: Another idea thats pretty much passed its sell-by date. Advances in fuel systems have virtually eliminated fuel evaporation loss, although it is true that there will be a small amount of loss by evaporation from older vehicles (1974 or earlier). You will get more savings by avoiding poorly fitted or missing filler caps to decrease evaporation loss.
Fuelling the Fire
They say: "Avoid filling up when you see a fuel truck at the garage if theyre receiving a fuel delivery, dont use the pumps. The delivery supposedly stirs up sediment in the stations tank that could be pumped into your tank, and dirty fuel will hurt your fuel economy."
We say: Again, something that had more relevance when tanks were made of metal rather than fibre-glass or other materials, and before the introduction of todays filters (both in the cars tank and at the garage).
They say: "The higher the fuel grade/octane, the better the fuel economy."
We say: Premium, or high-octane, rating is more a measure of the fuels resistance to abnormal combustions (pinging or knocking). Most engines are designed for a certain octane rating and putting a higher-octane fuel in wont improve efficiency as such.
Note though that running a premium-rated engine on lower octane fuel than it is rated for may well adversely affect efficiency.
They say: "Fill your tank when it is half full in order to minimize evaporation loss."
We say: Modern vehicle fuel systems have virtually eliminated fuel evaporation loss.
Tyred and Weary
They say: "Over inflating your tyres, beyond the standard levels, even to the tyres pressure range maximum, will improve your fuel economy."
We say: Just because under-inflated tyres waste fuel, it doesnt follow that over-inflating them beyond recommended levels will be even better. Not only does it make for a rougher ride, it also increases the chance of a blow-out under stress. Also, various tests have shown that, generally, over inflating tyres will have no measurable effect on fuel economy. The difference occurs when under-inflated tires are pumped to proper levels.
This fuel contains additives
They say: "This wonder pill, liquid, magnet, gizmo or whatever will dramatically improve your fuel economy by xx percent."
We say: Of course they wont. These products should all be treated with the if it seems too good to be true, then it probably is maxim. No truly independent and scientific tests have to our knowledge shown anything other than a negligible improvement gain from these gadgets. If one of these cheap devices actually did make a meaningful difference, the car manufacturers would be climbing over each other to grab hold of them.
They say: "Driving with the tailgate of your truck or Ute down will make the vehicle more aerodynamic, which in turn will help fuel economy."
We say: No, no, no. It sounds vaguely plausible in theory, but it has been proved incorrect by no less than those guys at Mythbusters. It doesnt work.
They say: "A dirty air filter will cost you - if your air filter is dirty, your engine will receive less air, but burn the same amount of fuel, throwing out the cars air/fuel ratio and making the engine less efficient."
We say: This was true once, but modern engines now compensate for the problems a dirty air filter would create. Todays systems will inject just as much fuel as the engine needs, and will adjust to changes in the amount of air being received.
So, there you have it, a not totally comprehensive list of fuel-saving tips that really dont stand up to the test. In the next article well be looking at a few things that really will work.
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