New Tyres

[Hedd_Jones]

Alan has allready hinted at the answer.

The 'true circumference' ( lets call it that so its easy to understand) is effected by a number of things, not least tyre construction and pressure.

There are 2 easy ways to get a pretty good answer.

1. Park the car on a hard surface and measure from the surface to the centre of the wheel. This radius will give you a pretty close answer.

2. Mark the tyre in pencil. Mark 2 lines on the ground (also in pencil) at start and stop for one turn. This will give spot on answer assuming you can mark and measure accuratley.

A tyre with a larger at rest diameter will not neccassarily give a bigger number. Though as a generalism you might expect it would, and probably does.

[Alan]

It’s because the canvas or steel carcass is very stiff compared to the tread rubber. For the same reason, Speedo accuracy isn’t as much affected as you’d expect by tyre wear.

[Duncan Grimmond]

"For the same reason, Speedo accuracy isn’t as much affected as you’d expect by tyre wear."
Speedo? accuracy? I understand that speedos are required to read 10% higher than real speed...easy to check with a GPS.

[Alan]

Not true. They certainly aren’t allowed to under read and there’s an upper limit too, but if you can do it exactly you are allowed.

[AustinWood]

It always used to be 'within 10%'.

However the PA in our 1929 fabric saloon is within 1mph all the way up to about 57mph and then reads a bit high. Checked against GPS and those flashing signs.

   

[Duncan Grimmond]

It would seem the vibration gets through at that speed though.. ;-)

[Tony Griffiths]

It would seem the vibration gets through at that speed though..  ;-)

No, that's the passenger shaking with fear!

[Robert Leigh]

... which is why the camera is shaking!

[AustinWood]

The passenger was me. We were on a single track road in the Lammermuir Hills with 45mph wind and higher gusts. It was not easy to hold the camera still!
The car itself was stable though.

[spcoates]

Blockleys give a hard ride. Changed to Longstones on my Chummy and ride and handling much improved.