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Speedometer Ratios

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Michael Gilligan05/03/2020 23:38:21
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23121 forum posts
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Posted by Neil Wyatt on 05/03/2020 14:15:02:

.

... let alone variations due to different types of tyre and state of inflation...

.

That brings back memories of a protracted debate in the ‘Reliant Sabre and Scimitar Owners Club’

... where it was concluded that [within reasonable limits] state of tyre inflation makes no difference to the speedo accuracy; because the circumference of the tyre is what matters, not the rolling radius.

MichaelG.

not done it yet06/03/2020 08:34:39
7517 forum posts
20 photos
Posted by Michael Gilligan on 05/03/2020 23:28:38:

Posted by not done it yet on 05/03/2020 22:36:05:

[…]

Incidentally, mechanical speedos are notoriously optimistic on speed indication. The legal range is to indicate up to +10% optimistic but with zero allowance for pessimism of the true speed. …

.

Which is [approximately] +/- 5% of the actual target reading ... it’s just that they shift the target to accommodate that ‘zero allowance for pessimism’

MichaelG.

No. Not plus or minus 5% at all. It is to prevent owners inadvertently speeding. A simple legal requirement that speedos provided must not indicate a lower speed than the actual speed.

Mike Poole06/03/2020 08:38:03
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3676 forum posts
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A tuned speedo is much cheaper than hot cams and gas flowing etc.

Mike

Michael Gilligan06/03/2020 08:42:53
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23121 forum posts
1360 photos
Posted by not done it yet on 06/03/2020 08:34:39:
Posted by Michael Gilligan on 05/03/2020 23:28:38:

Posted by not done it yet on 05/03/2020 22:36:05:

[…]

Incidentally, mechanical speedos are notoriously optimistic on speed indication. The legal range is to indicate up to +10% optimistic but with zero allowance for pessimism of the true speed. …

.

Which is [approximately] +/- 5% of the actual target reading ... it’s just that they shift the target to accommodate that ‘zero allowance for pessimism’

MichaelG.

No. Not plus or minus 5% at all. It is to prevent owners inadvertently speeding. A simple legal requirement that speedos provided must not indicate a lower speed than the actual speed.

.

dont know

I am perfectly aware of the logic, thank you

... It’s the practical application of statistical quality control that I was trying to discuss.

MichaelG.

not done it yet06/03/2020 09:27:34
7517 forum posts
20 photos

I wasn’t. It was the simple legal requirement - as set down in law. Hence my use of the word ‘legal’.

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