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@Sporky quick draw strikes again
apparently 4mm of tread on a brand new tyre causes circumference difference significant enough to be accountable for at least 6% speedo error
I'm not good at physics so I zoned out a bit and didn't really listen
Every alteration has a knock-on effect, even if you aren't intending it or aware of it.
Hours of fun available here & other sites https://www.wheel-size.com/calc/
The ACPO guidelines for speed enforcement had a table in them indicating the thresholds at which a fixed penalty notice was the recommended minimum sanction, and when a summons was the suitable response. These happened to be set at + 10% + 2mph for a fpn.
This was when cameras were everywhere before the funding from government got turned off under David Cameron.
Now, the guidelines are defunct, and a police officer can nick you for committing any crime - including driving 1mph over the speed limit, but then they always could, the ACPO guidelines were just guidelines.
They still however mostly allow you a bit of an allowance, even if it's only 2mph for calibration reasons though that would be very unusual.
Safest way to avoid a ticket remains... don't speed.
Side note, the old rules also included a lot of the popular myths about visibility etc. - now in Northampton I regularly see a speed camera than the local coppers use that is completely unmarked, just a grey van, and they do ANPR as well.
Lets use a fairly middle sized 235/60R16 tyre as an example (size made up)
16" = 406.4mm
60% of 235 = 141mm
So at 'standard' size, overall diameter is 688.4mm.
8mm is only 1.16% of a variation.
Even taking something like an old fashioned 165/80R13, the overall diameter is 594.2, and 8mm variation is 1.35%
However that is only the theoretical size, and tyre pressures have a far bigger effect on rolling size.
I have a bike which does 62mph at 4,000 rpm in top on a new tyre, which drops to 60mph with a worn out tyre.
So it's noticeable but rarely licence threatening.
I'm sure you can all calculate the sack size from that!
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