Measuring Speed re Speedometer +/-

mikey72

Flyboy152 wrote: »

Clearly, your theory tested wrong. :wall:

The car's speedometer is not allowed to underread. They are not supposed to overread either.

First bit is true, second bit?

Reading 70 you can actually have a real minimum speed of 57.

Formula is true speed/10 then add 6.25

so 57 +5.7 +6.25 = 68.95

I'm just building a car that has to have an iva, and a vosa approved test on this.

http://www.transportoffice.gov.uk/crt/repository/M1%20Inspection%20Manual%20May%2009.pdf


page 131

dusty64

It will cost you about £400 to calibrate your speedo to read correct speed. There is a company in Nottingham that the police use which will sort you out. A cheaper alternative is to buy a sat nav which is the next best thing (excluding time lapse). Lots of facts and figures being given on here but all calibration formulae is speed = distance/time. Quite straight forward really. I've never known a speedo to overread but it could happen. Most will underread by upto 10% (which is within tolerance).

mikey72

dusty64 wrote: »

It will cost you about £400 to calibrate your speedo to read correct speed. There is a company in Nottingham that the police use which will sort you out. A cheaper alternative is to buy a sat nav which is the next best thing (excluding time lapse). Lots of facts and figures being given on here but all calibration formulae is speed = distance/time. Quite straight forward really. I've never known a speedo to overread but it could happen. Most will underread by upto 10% (which is within tolerance).

I'll just put smaller wheels on until it reads high.

At 26mph true speed, it can overread by 9mph - 34%
At 57mph true speed, it can overread by 13mph - 22%


Can't underread at all.

Flyboy152

mikey72 wrote: »

First bit is true, second bit?

Reading 70 you can actually have a real minimum speed of 57.

Formula is true speed/10 then add 6.25

so 57 +5.7 +6.25 = 68.95

I'm just building a car that has to have an iva, and a vosa approved test on this.

http://www.transportoffice.gov.uk/crt/repository/M1%20Inspection%20Manual%20May%2009.pdf


page 131

A car's speedometer is not supposed to overread. It is allowed to overread, but there is no rule, regulation or even convention that says it is supposed to overread.

To say it is supposed overread, suggests that there is a deliberate procedure to show that the speedometer is indicating a faster than actual speed, when this is untrue.

Hintza

Flyboy152 wrote: »

To say it is supposed overread, suggests that there is a deliberate procedure to show that the speedometer is indicating a faster than actual speed, when this is untrue.

It probably is. But that will be from a cost and variance perspective, if the manufacturer decided to try for a spot on speedo, some would over read and some would under read. You can imagine the hoo haa if folk could prove the speedo was reading too low.

So to get the bell curve to work they need to shift the tolerances to read slightly higher than ideal.

mikey72

Hintza wrote: »

It probably is. But that will be from a cost and variance perspective, if the manufacturer decided to try for a spot on speedo, some would over read and some would under read. You can imagine the hoo haa if folk could prove the speedo was reading too low.

So to get the bell curve to work they need to shift the tolerances to read slightly higher than ideal.

Yes, if it was to say a +/-5% tolerence when manufactured it would be made to six sigma on limits of +10%, and -0% of true speed, so the speedos would be already on average 5% over when set correctly.
So it could leave the factory varying down to 0%, but euqually could vary up to +10% and still be legal and in spec.