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.


About Ralph Hosier

I love exploring everything the world has to offer, the fabulous beauty and intricacies of nature, the stunning majesty and grandeur of the universe, and the fascinating range of chocolates available from the local sweety shop. I have led a charmed life, sure there has been extremes, but the highs far outweigh the lows. I get paid for arsing about in very fast cars, I get to write about them and amazingly get paid for this too. My days are usually filled by making prototype and concept cars for car companies, a dream job. I have lived many of my dreams, worked all over the world, raced cars built by my own hand (and hardly ever crashed really badly), seen things and done stuff. But nothing compares to the love of Diana and my son Peter, beyond my greatest hopes. I am a chartered engineer, a member of the Institute of the Motor Industry (IMI), and of the Institute of Engineering and Technology (IET) and I am a member of the Guild of Motoring Writers. A pleasing fact is that there are now more letters after my name than there are in it ;) R.Hosier B.Eng(Hons) C.Eng MIET MIMI MGoMW
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