Rod Speed wrote:


"NY" wrote in message
o.uk...
"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?Â* 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.

Thats why I have a socket that takes the flattened end on the screw
https://www.walmart.ca/en/ip/black-j.../6000093043562
and a ratchet socket driver in with the other tyre tools. Corse
now I have organised that, I wont ever get another flat again.

I have just bought an electric scissors jack.

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.