Practical Edge – Skills Training

Category: Tips

Practical hints and tips

  • Getting better MPG

    I’ve tested a lot of cars over the years, proper scientific tests at vehicle manufacturers and I’ve noticed a few things about fuel consumption, yes it’s a fascinating topic and I regularly enthral audiences at parties with my stories….

    In many test over the years I’ve found the difference between the smoothest driving and harshest driving results in about 30% difference in fuel consumption, even in a convoy of test vehicles all starting at the same time and arriving at the same time. There is a real art to smooth efficient driving, so I’d recommend getting some expert tuition, it also reduced wear on the car and is generally safer. It involves looking ahead, reading the road and driving to a plan. It’s a good skill.

    There is a non-linear relation between speed and fuel consumption, most family cars seem to save up to 11% on motorways by going at 60mph instead of 70mph. But keeping a steady speed is just as important. The less aerodynamic your car is the more dramatic the speed effect is. The power needed to push a car through the air is proportional to the cube of the speed, so if you double the speed you need 8 times more power. That’s one of the reasons why land speed records are so difficult.

    Tyre pressures make a big difference, if you are supposed to have 30psi but you are running 20psi you will be using up to 10% more fuel and wearing them down faster. But going over the recommended pressure makes only a slight difference and can reduce grip. So check your tyres regularly. It’s amazing how many cars are running the wrong pressure, so few drives actually check.

    Tyre type also makes a big difference too, chunkier tread uses more energy. I tested a few options on 4x4s over the years and a chunky AT tyre uses about 20% more fuel than a summer road tyre on an average commute, and really chunky MT tyres can use over 40% more fuel. So look at the energy rating of your tyres next time you buy some.

    You may have heard that your should remove all the unnecessary junk from your boot, well although vehicle weight makes a difference it’s only a very small difference, and taking 50kg out of a 1500kg car is negligible. Unless you are actually on a race track.

    Combustion engines don’t run at their best when cold, modern petrol engine heat up fairly quick but diesels can take a bit longer, so plan ahead and combine several trips in one so you are not starting with a cold engine.

    If you have a manual gearbox then make sure you are in the right gear, which is not necessarily the highest gear. An engine is at it’s best efficiency at the RPM where it makes peak torque, somewhere between that and maybe 1500 RPM works best for many cars, but try a few runs an vary your method to see what works for you.

    It may come as a surprise but servicing is really important, using good oil in the engine and transmission makes a big difference, I once picked up a ‘spares or repairs’ Peugeot 306 which gave 25mpg on my commute until I changed the tar like old oil for new, it jumped to 40mpg, the most dramatic difference I have encountered. Proper servicing is actually an investment that pays for itself.

    The old argument about air-con vs having the window open is a bit complicated as it depends on the weather and how you are driving. The drag of an open window is small compared to overall drag even up to 60mph, above that it becomes more significant. Whilst air con used in the UK can use an average of 3 to 9 bhp on a typical family car, compared to an average of 25bhp used to maintain 60mph on a motorway, so it makes a significant difference. At lower speeds like city traffic where you will use less average power the air con still uses the same power, so the difference is more noticeable, maybe up to 14% more fuel used for a town commute.

    Roof boxes add drag, but again this becomes increasingly important as the speed rises above 50mph, in city traffic it makes little difference but on the motorway it will definitely consume more fuel, so if you are not using the roof box then either take it off or drive a bit slower to save fuel.

    So basically take it easy, plan ahead, service your car and check the tyres.

  • Brake judder

    Brake judder can be caused by a number of things, sometimes difficult to accurately diagnose. Most often it will be because the disc does not run true in the calliper, this could be because the disc has warped due to excessive use (quite rare) or was fitted incorrectly (more common). Another cause on older car is where the disc has corroded unevenly, particularly if the car has stood for a while, sometime the area under the pads remains untouched whilst the rest of the disc rusts.

    Fitting discs is not just a case of bolting them on and hoping for the best, any slight unevenness where the hub meets the disc can tilt the disc slightly, a small error near the centre makes a big difference near the edge of the disc, anything over 0.1mm of variation is a problem. If there is any slight tilt in the disc it will gradually worsen as it wears unevenly, initially it might feel fine but typically it becomes a big problem between 2000 and 4000 miles after the discs have been changed.

    For this reason after you have removed the old disk it is vital to clean up the mating surface, initially clean off all the dirt from the area then using a flat edge ensure the mating surface is completely flat. It should only take a moment to do but makes a big difference.

    It’s remarkable how many ‘professional’ places don’t do this.

  • Lying cars

    Why your car is lying to you.

    We rely on some of the things a car tells us, like speed for instance, in order to stay safe and also to stay on the right side of the law. So it might be odd to hear that many things a modern car tells us are in fact quite deliberately wrong.

    Here’s an experiment for you if you have a flash car; look at the trip computer, fill the fuel tank then make a note of ‘distance to empty’ or ‘range’, and after a long drive when its nearly zero make a note of ‘average MPG’ too. When you fill the tank you can work out the real MPG. What you will probably find is that the reported MPG is rather optimistic, but its not because the system is inaccurate, modern systems are really rather good at being accurate. Most systems are very precisely over optimistic by up to 10%, although I have seen certain Teutonic luxo-barges be out by 20%. Obviously this is to ensure the customer feels better about their consumption of the earth’s natural resources.

    The range calculation has a different story to tell. Again it can be very accurate, but because some customers ‘chance it’ it says zero when the car still has a few miles left in it. This is more a matter of self preservation than conning the driver, if a modern car runs out of fuel all sorts of bad things happen such as catalyst or fuel pump failure. But if you are driving consistently you will probably find that when range has gone down by 10 miles you have in fact actually travelled 10 miles.

    Now here’s the funny thing, if anyone actually compared the MPG and range info they would see the two don’t tally. But of course you would have to be pretty bored to do that.

    You probably know the speedo always reads slightly higher than the real speed, but do you know why? Many years ago when gauges were made of brass and springs, they were not very accurate which is a problem if you don’t want to be arrested for speeding, so laws were introduced to tighten things up. The law had to allow for the inherent inaccuracies of the measurement method, in the UK this means the gauge is allowed to read anywhere between the true speed and 10% higher, but because there are variations in accuracy due to production tolerance manufacturers tend to play it safe and aim for the middle of the allowable range. So most read 5% over.

    But again the gauges don’t agree with each other. On most cars the odometer is fairly accurate, so if in that remarkable moment of boredom you where to divide the change in mileage by the time taken you would find the true speed. Although to be fair it would be a lot easier to look at a GPS unit.

    Other gauges take an even greater liberty with the truth. Years ago some cars had oil pressure gauges, readings were read with the same intensity that a fortune teller reads tea leaves, often adverts for second hand cars read something like ‘good oil pressure’. But oil pressure can vary between one engine and another as they trundle off the production line, there is nothing wrong with this; some engines last a lifetime with really low pressure. Unfortunately some owners became a bit paranoid about the minute flickerings of that little gauge and sent their cars back, so drastic measures were taken. For instance if you bought one of the last Jag V12s the oil pressure gauge was in fact only connected to the pressure switch and a resistor, so as soon as the engine started it stayed pointing resolutely at the middle of the range, very comforting. They were by no means the only manufacturer tackling the problem imaginatively. But as soon as computer controlled dash instruments hit the main stream in the 90’s the standard method became to make all the gauges read something nicely reassuring unless there was an actual real problem that needed the driver to take action.

    Its the same with the temperature gauge, as the real engine temperature fluctuates the gauge reads a nice steady ‘normal’ and only climes out of its comfort zone if the car thinks it’s in immanent danger of exploding.

    Now, you might feel rather cheated by all this, but actually for most drivers its probably for the best. If you don’t happen to understand that oil pressure and coolant temperature do vary a lot then you might get quite anxious as the gauges dance about. By only alerting the driver when there is a genuinely something to worry about allows them to concentrate on driving but still take action when necessary. And if the speedo reads a little high then you wont get flashed by cameras if you stray a few MPH over the indicated limit.

    So although its lying to you, it means well.

    Mostly.

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