I have had a Sat Nav for over a year now. It is one of the best gadgets I have ever purchased. The amount of time I spend lost in cities has almost reduced to almost zero. I confidently navigate complex motorway interconnections. I find remote destinations.
One of the many features the Sat Nav helpfully provides me with is a measure of my current speed.
I have noticed that the speed the Sat Nav thinks I am travelling at is slower than the speed that the car dashboard speedometer thinks I am travelling at. I have been wondering why this is.
The Sat Nav measures speed by recording the time it takes to travel between fixed points calibrated by satellites. This should be fairly reliable (except on steep hills, when the distance actually travelled will be further than the Sat Nav thinks, so the real speed will be greater).
The speedometer measures the speed of rotation of the wheels. In order to assess actual speed, it must make assumptions about how big the wheels are. If these assumptions are wrong then its calculation of speed will be wrong.
Car manufacturers tend to 'play safe' by calibrating speedometers high. This also means that the number of miles "on the clock" will be higher than the number of miles actually travelled, and fuel efficiency calculations will appear more favourable.
Furthermore, as the car tyres wear down there will be a gradual increase in the speed that the speedometer calculates. This is because the distance actually travelled per rotation becomes less as the tyre wears.
How important is this difference?
Well, my Sat Nav currently measures 67 mph when my speedometer displays 70 mph. So if I am holding my speed, according to the Speedometer, at 70 mph on long motorway stretches, it is actually taking me 5 minutes longer than I am expecting for every 100 miles travelled.
That may not seem much in itself, but over the course of a year, this could equate to the equivalent of a whole working day. Over 25 years that is roughly a working month of your life wasted.
So that's two good reasons why changing worn tyres could actually help you live longer!
Edgar Bolton. 2013
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