Dear LPG. 

I am sure even the drivers who read this website have rarely, if ever, really thought about this aspect of driving very much in their lives, but the subject got my brain into gear when I thought about it…

A mathematically minded friend and I were talking recently about the news that all the streets in the Boroughs of Camden, Islington, Hackney, Haringey and Tower Hamlets now have speed limits of 20 mph. 

We were having a bit of a complaining session because while this will save lives, which is a positive thing, the fact that our petrol is heavily taxed and the slower you drive, the more of it you use is a downside that the consumer/ driver picks up the tab for. It is also nigh on impossible to get from anywhere to anywhere else without working your way through the many and forever-changing one-way systems, no-entry roads, school time blockades, road works and other temporary and permanent diversions that litter our boroughs.

My friend started thinking mathematically about something few drivers consider when changing or even pumping air into the tyres on their cars. Tyres come in different sizes, but the measurement most commonly used to define them is that of the tyre's rim and not the outside circumference. But that is not the bit that touches the ground. He discovered that the tyre's pressure has to affect the size of the circumference and hence the amount of time it takes one to turn a complete rotation. 

A car's speedometer has to work on the number of times the tyre turns, and the size of what the internet tells us, which is called the 'rolling circumference' in the trade, must vary a bit. We found a website that suggests that a 2-inch variation in psi (how much air is in the tyre) can change the length of the circumference by 1/32 of an inch, which means you only need 32 revolutions of the wheel to make a difference to the length of road covered. Then we worked out that there are only 63,360 inches in one mile, and when you put that all together, there could be a 19.8-inch level of inaccuracy for every mile we travel, and we were only thinking about two psi.  

We then put our heads together and remembered that old mathematical formula. 

speed = distance/time

Which flagged the possible discrepancy in the difference that could affect a car's speedometer reading capability when it comes to every mile you travel.

According to the internet, it is negligible, but it gets you thinking about your speedometer and its accuracy when you are motorway driving, if nothing else… 

AM, Lee 

AM share some information but LPG would like to reiterate that a lot of it comes with a selling agenda included and reminds you to focus on the information rather than the sales pitch…

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