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[Kevin Wood]

When I take over the world

Law 1

All speedos to 1% accuracy

*********** politicians!

I disagree. I would require the speedos on all cr@ppy "sit up and beg" cars to under-read so when they think they're mimbling along at 45 MPH on a clear stretch of "A" road they are actually doing a respectable speed.  

Kevin

[Martin_1962]

When I take over the world

Law 1

All speedos to 1% accuracy

*********** politicians!

I disagree. I would require the speedos on all cr@ppy "sit up and beg" cars to under-read so when they think they're mimbling along at 45 MPH on a clear stretch of "A" road they are actually doing a respectable speed.  

Kevin

[Martin_1962]

When I take over the world

Law 1

All speedos to 1% accuracy

*********** politicians!

Which I supect is not a viable option

Consider that a 225/55/16 tyre has a circumference of 2054mm with a tread depth of 8mm......at 2mm tread depth the circumference is now 2016mm......Thats a 1.85% change just through tyre wear!

Then add in component tolerances etc......and you actually find that the Current speedo setup is pretty dam good!

6mm wear = 12 mm dia = @ 38mm - thought it was a bit

With modern electronics they could get accurate at half worn so within +-1% at all wear. This no underread in any circumstance is a bit silly - it is not like it is a safety critical part of the car - safer to have no speedo then people will not aim for a target.

[Martin_1962]

What I was trying to say no point in an instrument unless it is reasonable accurate and 3mph at 60mph is pretty bad, should be no worse than 59 or 61 depending on tyres.

Last time I had an accurate speedo was running the higher geared axle of two on one speedo gearing

[Kevin Wood]

You've got to remember that the rules were written when a speedo was a couple of rotating magnets in the rough vicinity of an ali disk and a spring. Lucky if it's within 10% let alone 1%.

When I SVA'd my kit car it was tested on a  rolling road at speeds up to 70 MPH. At an indicated 70 MPH the true road speed was allowed to be between 57 and 70 MPH. I think the allowance is 10%+6.25 MPH  

Kevin

[Martin_1962]

You've got to remember that the rules were written when a speedo was a couple of rotating magnets in the rough vicinity of an ali disk and a spring. Lucky if it's within 10% let alone 1%.

When I SVA'd my kit car it was tested on a  rolling road at speeds up to 70 MPH. At an indicated 70 MPH the true road speed was allowed to be between 57 and 70 MPH. I think the allowance is 10%+6.25 MPH  

Kevin

Funny really - at least yours will be OK

[edwardmickey]

The calibrated speedo on our police vehicles are checked at least once a month - I've never had to defect any due to inaccurate - no matter what the tyres are doing.

I assume that the speedo should be accurate for life - bar collisions..... (ouch - polaccs - nasty horrible work)!!!!