"dennis@home" wrote in message
...
On 13/02/2019 21:39, alan_m wrote:
On 13/02/2019 18:12, Robin wrote:

depends on wheel/tyre of course but I was reckoning on tread going from
8mm to 2mm on a tyre of around 180mm circumference.


What about different minor variants of the same basic car? It wouldn't
be unusual for them to be fitted with either,say, 16 or 17 inch wheels.
Do the manufactures adjust the calibration for different factory fitted
wheels or do they rely of range they allowed on the speedo readings?

What about the reading within spec with a space-saver wheel on one corner



You aren't supposed to drive with the space saver on the front so that
removes front wheel drive cars from the equation.

They nearly always recommend swapping the wheels so the space saver is on
the rear.

People don't of course because they are too idle.

I've not actually heard of that recommendation, though I can see that it
makes sense. It does take a lot longer, because you have to make two
manoeuvres instead of one:

- spare on back to free up a good tyre
- good tyre on front in place of punctured tyre

Mind you, a lot of the time of changing a wheel is initial stuff like
removing stuff from the boot onto the back seat to lift the boot floor to
get at the spare and the jack.

With modern scissor jacks (which have almost no ground clearance for the
handle to turn without grazing your knuckles, it is a thankless task.

The last time I had to change a wheel was about 6 months ago when I caught
the inside of a tyre (almost brand new) on the edge of the road surface that
stood proud of a rut beside the road, when an oncoming tractor who should
have given way to me bullied his way forward so I had to veer off the road
to avoid a collision. The tyre held up for another half-mile till I got home
and parked, but a few minutes later a neighbour said "do you know you've got
a flat tyre". That was a waste of £40: there was a huge gash in the inside
wall, a *long* way from the tread so no quibble: the tyre was unrepeatable.