How to check if you have been caught in a police speed trap

Harry Bloomfield said:
The rolling radius of a tyre, varies very little between new and worn out.
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Doesn't matter. As the tyre wears its circumference reduces, meaning it must rotate faster for the car to do the same speed. As the speedo is driven from the rotation of the wheels it will progressively over-read as the tyre wears

Put another way, if you have a set of brand new types on a car and you tow that car at 100mph (let's say, using a truck whose speedo reads 100mph) and the car speedo reads 100mph, then stop and swap the new car tyres for worn out ones and again tow the car at 100mph on the truck speedo, the car speedo will read higher (say, 103.4)

This is permitted, and means if you drive the car at an indicated 100mph you will be doing up to, but not more than 100mph
 
robinbanks said:
Doesn't matter. As the tyre wears its circumference reduces, meaning it must rotate faster for the car to do the same speed. As the speedo is driven from the rotation of the wheels it will progressively over-read as the tyre wears

Put another way, if you have a set of brand new types on a car and you tow that car at 100mph (let's say, using a truck whose speedo reads 100mph) and the car speedo reads 100mph, then stop and swap the new car tyres for worn out ones and again tow the car at 100mph on the truck speedo, the car speedo will read higher (say, 103.4)

This is permitted, and means if you drive the car at an indicated 100mph you will be doing up to, but not more than 100mph
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I've read numerous reasons for the quite tiny variation, but I have carried out my own experiments. Comparing digital dash speedo, to satnav, as my tyres wore from new to needing replacement - there was no distinguishable difference, between new and worn out. My digital speedo is fed from the ABS pulse at the right front wheel.

I do agree with you that common sense does suggest that there should be that difference of around 3mph, but in it simply doesn't work like that - tyres, on the road, are not circular - they are a circle, with a flat bit at the bottom.
 
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Harry Bloomfield said:
I've read numerous reasons for the quite tiny variation, but I have carried out my own experiments. Comparing digital dash speedo, to satnav, as my tyres wore from new to needing replacement - there was no distinguishable difference, between new and worn out. My digital speedo is fed from the ABS pulse at the right front wheel.
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You sure live the high life Harry.
 
robinbanks said:
I suspect your attempts to observe it are becoming lost in measurement inaccuracies
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Having given it more thought, as I pointed out above - a tyre on the road is not a perfectly round object, they are flat at the bottom. The cords are buried deep in the rubber, the rubber is compressible. What sets the rolling diameter for the most part, is the reinforcing cords, the rubber simply flexes and compresses, as the tyre rotates. Hence, tyre wear, makes much less difference to the accuracy of the speedometer, than you are suggesting.

What does make a difference, is changing the tyre, for one of a different diameter.

A rigid wheel, as it wears, will behave exactly as you suggest, and cause an error.
 
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