Taxi Driver Online

just bought a new second hand tyre today too

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Think of how stupid the average person is, and realize half of them are stupider than that.
George Carlin

I had to think about wee eddies post for a while 'cos it didn't seem correct, but he's actually right.

The diameter might change with less pressure but the circumference remains the same A tyre is a pretty solid lump of rubber, in that it doesn't stretch much, if at all. If you look at one on the shelf in a fitters it's the same size circumference as one fitted on a rim with 30psi in it. And not forgetting a tyre isn't all rubber, there's all the steel banding wrapped around it which will stop the tyre becoming larger (or smaller) as well.

The only thing that will change the circumference would be tread wear. So if 'people are paying through the nose on some journeys' it's not because of tyre pressures, which makes the councillor even more ignorant than first thought !

In a perfect circle the circumference is worked out by multiplying the diameter by PI. So any change in diameter will affect the circumference. However a tyre is not a perfect circle in that the pressure inside will change. The one constant being the steel banding inside the trye. I don't think that the length of the steel band will flex at all so the length of that will remain constant.
If the depth of tread on a new tyre is taken as 8 mm and the depth on tread on a barely legal tyre is taken as 1.6 millimeters then the wear from new would be 6.4 mm or 12.8 mm on the diameter. this will mean a change in circumference over time of appoximatly 40 mm. The rolling circumference of a typical new tyre fo size 195-65-R15 is 1935 mm so to cover 1 km the tyre would revolve 516.8 times when new. when it is at it's minimum it would revolve 527.7 times, an extra 10.9 rotations or a distance of 21.1 meters compared to a new tyre, or 23 yards in old money. How many meters have drops of 23 yards? You would require a journey of at least 6 km in most boroughs before you gained 1 drop on the meter.
God my head hurts now.

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Grandad,

All metres work off the gearbox, and CC is guilty as charged as he has openly stated his is knackered

grandad: The circumference is the distance around the outside of the tyre. The method you use to calculate is is irrelevant and doesn't alter it in any way.

As the wheel rotates, it travels along the road by exactly the distance of the Circumference. Unless you are doing wheelies (more) or skidding (less).

Any changes in the shape of the wheel are irrelevant and do not alter the Circumference.

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Don't dream it ~ Be it.

Did you actually read the whole of my post?

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Grandad,

Under inflation of tyres would make very little difference to the meter reading if any. It would have to assume you had perfect pressures at the time of calibration, and that your speed sensor is working perfectly. This is why you have a measured mile for calibration, rather than running off the odometer. There are new systems out that are using lasers and are very accurate - something like 0.001.

In my experience, around 60% of odometers are wholly inaccurate - most are around 0.1 - 0.2 below. As speed sensors wear, they become inaccurate or erratic meaning that on a mile you can measure vastly over, or vastly under. The worst I have seen was a sensor measuring 0.8, 0.6, &1.1 on three consecutive measured mile runs where the sensor was worn.

If you want to keep earning the right amount, have your meter calibrated at least once every 6 months. That will make more of a difference to your income than tyre pressures! You could possibly gain 1 or 2 pence a week if busy enough - weigh that up against two tyres, and you would be pretty mad to try it. There are lots of myths around the taxi industry, this I would say is one of them.

easy to check if your LA has a measured mile, just try it

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Of all the things ive lost, i miss my mind the most

When I said the diameter would change I meant if measured from top to bottom whilst on the car. A correctly inflated tyre might be 36 inches across but an under inflated one 30 inches, but the circumference hasn't changed as the missing diameter is 'squashed' along the bottom of the tyre. It's just no longer a perfect circle, think of it as turning a circle into an oval - the distance around the outside edge remains the same.