What speed am I doing

bryanb

MarkTed wrote: »

I don't believe on Speedos on a 100%.. Perhaps I might view them as useless.. Rather use my speedometer.

You mean we're not discussing swimwear?

smudger1964

The speedometer on my car reads 30 but my true speed is 40 guess I must have one of the only cars with a speedo that reads less than im doing.I always have to do at least 10 mph less than the speed limit

Notmyrealname

shaun_from_Africa wrote: »

The law actually gives a far wider figure than this.

No it doesn't. The law doesn't give even 1MPH. The ACPO issued guidelines for EXCEEDING the limit as you stated which police follow but don't have to.

Provided that the indicated speed is within 10% + 6.25 MPH of the actual speed then the car speedo is within its permitted accuracy. (but as already mentioned, this is only for over reading).

Wrong. Construction and Use regulations allow a car speedo to OVERREAD by no more than 10% and a HGV/Coach to overread by no more than 2.5%. They cannot underread at all.

So if your speedo is showing 30mph, you could in fact be going as slow as 20.75mph, although in reality I doubt if many instruments would be this far off.

If that is the case your speedo does not meet the C&U regs and is therefore illegal.

Notmyrealname

smudger1964 wrote: »

The speedometer on my car reads 30 but my true speed is 40 guess I must have one of the only cars with a speedo that reads less than im doing.I always have to do at least 10 mph less than the speed limit

Your speedo is legally defective as it is not allowed to under read and if you stated that to a police officer and they decided to get VOSA to check it, you could be prosecuted under a Construction and Use offence.

fivetide

smudger1964 wrote: »

The speedometer on my car reads 30 but my true speed is 40 guess I must have one of the only cars with a speedo that reads less than im doing.I always have to do at least 10 mph less than the speed limit

Has it got aftermarket or altered wheels on it? You could have thrown the rolling radius right out.

5t.

Bongles

Notmyrealname wrote: »

Wrong. Construction and Use regulations allow a car speedo to OVERREAD by no more than 10%.

Have you got a reference for that? This bit of the C&U Regulations doesn't say anything about accuracy. This came up in the other thread the OP referred to and someone else suggested 10% + 6.25mph is allowable for overreading, although as far as I could see in the two sets of rules the C&U regs referred to, Community Directive 75/443 and ECE Regulation 39 (first hit on Google is a pdf download), the limit appeared to be 10% + 4kph. Either way, it does seem to be allowed to overread by more than 10%.

shaun_from_Africa

Notmyrealname wrote: »

No it doesn't. The law doesn't give even 1MPH. The ACPO issued guidelines for EXCEEDING the limit as you stated which police follow but don't have to.


Wrong. Construction and Use regulations allow a car speedo to OVERREAD by no more than 10% and a HGV/Coach to overread by no more than 2.5%. They cannot underread at all.

I take it that you don't believe that the "The Motor Vehicles (Approval) Regulations 2001" qualify as UK law then? (Schedule 3 para 19-3)

http://www.legislation.gov.uk/uksi/2001/25/schedule/3/made

2. For all true speeds up to the design speed of the vehicle, the true speed shall not exceed the indicated speed

3. For all true speeds of between 25 mph and 70 mph (or the maximum speed if lower), the difference between the indicated speed and the true speed shall not exceed—
V/10 + 6.25 mph
where V = the true speed of the vehicle in mph.

Underreading not permitted, overreading up to 10% (or V/10) + 6.25mph is allowed.

So where exactly was my earlier post wrong?

Lum

GPS is only really accurate if you are driving in a straight line on a flat road at a constant speed with a good view of the sky and no tall buildings around you.

Once you get to such a situation, your satnav and iPhone GPS should match, and you can compare this figure to what is on your speedo and use it to determine how inaccurate your speedo is, and therefore how much extra you can get away with while remaining legal.

Do bear in mind that the speedo error may not be consistent, or even linear, so you really ought to do the test at a variety of speeds. I normally just do 50, 60 and 70 because for 30 and 40 I don't want to be calculating and remembering differences just to squeeze out a few more MPH in an urban environment.

Also bear in mind that as you tyres wear out your speedo will overread even more, so it's important to regularly recalibrate it and particularly important to recalibrate as soon as you fit new tyres.

Hurri

Lum wrote: »

GPS is only really accurate if you are driving in a straight line on a flat road at a constant speed with a good view of the sky and no tall buildings around you.

As I stated above, this is *not* how typical SatNavs calculate speed.

"GPS receivers typically use carrier phase rate (doppler effect) to calculate speed, and is not just a simple distance/time calculation."

fivetide

Hurri wrote: »

As I stated above, this is *not* how typical SatNavs calculate speed.

"GPS receivers typically use carrier phase rate (doppler effect) to calculate speed, and is not just a simple distance/time calculation."

Sorry but your user name is tailor made for this thread! :beer:

That is all.

5t.