Speedo Error Calculation

[BBB61]

If a speedo had been tested and found at an indicated 30mph to be a true 25mph, what would be the error in percentage terms?

 

[Paul Wakefield]

20%

Paul

 

[wildbeeste]

or 16.666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666667%

Depends on which way you calculate it.

 

[Paul Young]

I'd go with Paul & say the speedo is reading 20% over...

 

[PhilBolton]

the speedo is reading 20% faster than you're travelling

OR

You're travelling 16.67% slower than indicated

Phil

 

[BBB61]

Cheers gents, that's the same figures I came up with. I asked because a certain company has told me that a true speed of 25 with an indication of 30 is a percentage error of 1.3!!

 

[thornley]

If your speedo is this far out is it illegal?. My understanding is it must be accurate +/- 10%, am I right?.

 

[Flipfly]

I don't think this is correct. I know the police offer a +10% usually to allow for inaccurate speedo readings but I don't think that's a legal thing. I would assume that it has to read no more than 10% over but if it reads less there is no issue as you would never speed (does this make sense??).

 

[GSmonkey]

If your speedo is this far out is it illegal?. My understanding is it must be accurate +/- 10%, am I right?.

Construction and use regs (don't have a reference??) call for +10%, -0% i.e. the speedo can never show you going slower than you actually are.

Because it is impossible to design any instrument to have 0% error in either direction, they will target the speedo to over read, then allow say a +5%, -4% error, which is realistic to manufacture.

Therefore the 'average' speedo should over read to a degree on new tyres, then even more on worn tyres.

 

[BBB61]

Its EU regs that count now with respect to speedo accuracy. They allow for a maximum of 10% + 2.5mph over-read. The regs also state that speedos can not under-read.

 

[thornley]

Cheers I'm now better informed.

 

[Flipfly]

Wow, I never knew that. Why is it unacceptable to under read your speed. Why would the police allow you 10% for your speedo if it isn't allowed to under read??

It's a mystery of the EU