On 08/06/16 14:10, NY wrote:
"The Natural Philosopher" wrote in message
...
So speedos are DELIBERATELY designed such that no combination of tyres
pressures and loads will ever result in them under reading, and they
can in fact overread by MASSIVE amounts and still be 'street legal'

So the next time you are sitting in the fast lane at what you think is
70, bear in mind you might be doing no more than 58mph...

Irrespective of how that is arrived at, unless you are using GPS or
doppler you WILL be counting rotations of prop shaft or wheel, so the
tyre loading/pressure/make/wear issues are all still in the mix.

By comparison with a GPS speedo app on my mobile phone, my car's speedo
seems to over-read by a near-constant 5 mph over the 20-80 range, so I
know that I can still be legal when the speedo reads 35, 65, or 75 (for
30, 60 and 70 limits respectively). I gather that normally the
over-reading is proportional to the speed, so a constant error of +5 is
unusual.

How much error do you get by tyre wear? For 225/65R17 tyres, the outside
diameter is 2 * 65% of 225 mm plus 17 inches which is 725 mm.

If tyres have about 8 mm tread when new and it reduces to 1.6 mm before
replacement, that means that the error is 2*(8-1.6)/725 = 1.7%

You will get more radius variation than that by putting the MIL in the
back seat.

Tyres do not exhibit a 1:1 correlation with the road surface. They
creep. And they stretch under pressure.


--
Canada is all right really, though not for the whole weekend.

"Saki"