Speedometers offset forward slightly?

[The Learner Driver]

I'm learning to drive and a mate said that in most cars the speedometer is offset slightly, so if it shows 30mph then you're actually doing around 26. Is that correct?

Is it the case for certain car makes or is it some kind of unwritten rule that everyone follows?

Thanks

[Number4]

It is completely true. I have never driven a car that does not do this.

If you have a GPS, compare the GPS speed to the speedometer.
Your GPS should be 99.99% accurately describing your speed, unless it is a dud.
The speedometer will most likely be over reporting by 10%.
The law requires speedometers to be accurate to +/- 10%. Most manufacturers err on the side of caution.
The old bill can't book you if you are within this speed range. eg 77mph is just ok on a dual carriageway.
Changing wheel and tyre size will effect your gearing and the speedometer reading.

[Mike Harte]

Originally Posted by Number4

It is completely true. I have never driven a car that does not do this.

If you have a GPS, compare the GPS speed to the speedometer.
Your GPS should be 99.99% accurately describing your speed, unless it is a dud.
The speedometer will most likely be over reporting by 10%.
The law requires speedometers to be accurate to +/- 10%. Most manufacturers err on the side of caution.
The old bill can't book you if you are within this speed range. eg 77mph is just ok on a dual carriageway.
Changing wheel and tyre size will effect your gearing and the speedometer reading.

I suggest that you consult a lawyer if you believe that you will not be prosecuted if you exceed the national speed limits by 10%. The police may use their discretion to not report you for the offence, but they may also choose not to do so.

And speed cameras are now being set to as low as 1 mph over the limit at certain points, at which you will receive a speeding ticket. It's a good money raising programme, and more areas are lowering the "discretion speed".

[Number4]

Originally Posted by Mike Harte

I suggest that you consult a lawyer if you believe that you will not be prosecuted if you exceed the national speed limits by 10%. The police may use their discretion to not report you for the offence, but they may also choose not to do so.

And speed cameras are now being set to as low as 1 mph over the limit at certain points, at which you will receive a speeding ticket. It's a good money raising programme, and more areas are lowering the "discretion speed".

I was wrong, it is 10% +2mph, OR at the officers discretion.

From the Crown Prosecution Service's website

http://www.cps.gov.uk/legal/p_to_r/r...nalty_notices/



Speed Enforcement

The Association of Chief Police Officers (ACPO) issued revised speed enforcement policy guidance in 2013. It suggests that enforcement will normally occur when a driver exceeds the speed limit by a particular margin. The particular margin is normally 10 per cent over the speed limit plus 2 mph. The guidance sets guidelines for when it would be appropriate to issue a fixed penalty notice or for the driver to attend a speed awareness course, and when it becomes appropriate to issue a summons. These are guidelines only and a police officer has discretion to act outside of them providing he acts fairly, consistently and proportionately.
In summary the guidelines are:
Speed limit: 20 mph
ACPO threshold for:

• a fixed penalty or a Speed Awareness course: 24 mph
• summoning: 35 mph

Speed limit: 30 mph
ACPO threshold for:

• a fixed penalty or a Speed Awareness course: 35 mph
• summoning: 50 mph

Speed limit: 40 mph
ACPO threshold for:

• a fixed penalty or a Speed Awareness course: 46 mph
• summoning: 66 mph

Speed limit: 50 mph
ACPO threshold for:

• a fixed penalty or a Speed Awareness course: 57 mph
• summoning: 76 mph

Speed limit: 60 mph
ACPO threshold for:

• a fixed penalty or a Speed Awareness course: 68 mph
• summoning: 86 mph

Speed limit: 70 mph
ACPO threshold for:

• a fixed penalty or a Speed Awareness course: 79 mph
• summoning: 96 mph

[Dave Brand]

Don't confuse guidelines with law! Speed limits are just that, limits which are legally enforceable. As Mike has said, you can be prosecuted for 1 mph over.

As for speedo readings, the law states that they may over-read by up to 10%, but must not under-read, so manufacturers build in a little bit of over-read just to be on the safe side. My DS3 over-reads by 2mph throughout the speed range, as verified by GPS.

A good rule for learner drivers: ignore anything your mates say!

[graham bahr]

most cars over read by 4mph, which im my view is no bad thing

[grantp]

Every car I have checked in the past 20 years or so overreads (because the manufacturers are liable if it underreads) by 2 to 4 mph through the range. Mostly it's about 2 to 3 mph - or the width of the needle if there is one - so the margin for error is small.

Ignore what mates say and make your own decisions when the time comes. That time is unlikely to be in the first year or two after you have passed your test since the effects of any penalties you receive are greater during that time.

If you are driving a car that has been modified, especially in terms of wheel and tyre sizes, be extra careful to fully understand whether the mods have had any effect on the indicated speed.

[Adam43]

Stick to below the limits. Concentrate on the road and the potential hazards. This is especially important when in areas, like towns, where you are around things outside your control. For example, over 30 in a 30 is idiotic.

FWIW, I find most modern cars read a consistent mph higher than the real speed, not a percentage. Most old cars tend to be a percentage, possibly because of modifications or different wheels or tyres.

[DAVID PATERSON]

My 2012 Ford Falcon reads just 1km/h over the actual speed, but my 2001 Isuzu F-series reads 10% over the actual speed. Both tested with GPS.

[wnut]

Spare a thought for the motorists of Victoria Australia, you know where the Australian GP is held:

2 km/h tolerance for fixed speed cameras
3 km/h tolerance for mobile cameras

Are these cameras capable of this accuracy, or is it just revenue raising?




http://www.heraldsun.com.au/news/vic...6396b8e3197d5e

[DAVID PATERSON]

Originally Posted by wnut

Spare a thought for the motorists of Victoria Australia, you know where the Australian GP is held:

2 km/h tolerance for fixed speed cameras
3 km/h tolerance for mobile cameras

Are these cameras capable of this accuracy, or is it just revenue raising?




http://www.heraldsun.com.au/news/vic...6396b8e3197d5e

I'm paranoid as soon as I cross the Murray, I dare not go near the speed limit.

It would be interesting to compare data with other developed countries and see if there is a correlation between speed limit enforcement levels and road tolls...

[The Learner Driver]

I guess it's true then and my mate was right, not that I will tell him ha ha.

Is it the same in Europe or can it differ to the UK? I've seen on Wikipedia that in Germany the speed limit can be up to 81mph so if it's the case there as well then they could be reading nearly 90 on the speedo.

[Dave Brand]

Originally Posted by Adam43

FWIW, I find most modern cars read a consistent mph higher than the real speed, not a percentage. Most old cars tend to be a percentage, possibly because of modifications or different wheels or tyres.

Older cars had mechanical speedos, so the reading was directly related to the speed of rotation of the cable; the drive gear was slightly different to what was required to give a true speed reading. Modern speedos are electronic, so they can be programmed to read whatever the manufacturer decides.

The Chevrolet Impala we rented in the US last year had the facility to switch the speedo reading to km/h; the dial had only one set of figures . . . want to see anything above 140 km/h & you have to use the digital display.

American car speedos, in my recent experience, read true speed, as checked against GPS.

[JohnD]

The law on spedoomnetrs accuracy is set out in United Nations "UNIFORM PROVISIONS CONCERNING THE APPROVAL OF VEHICLES WITH
REGARD TO THE SPEEDOMETER EQUIPMENT INCLUDING ITS INSTALLATION" See: https://www.unece.org/fileadmin/DAM/...gs/r039r1e.pdf

The speedo may have up to 10%+/- accuracy, but may NEVER underread. So they always overread.

John

[The Learner Driver]

Originally Posted by Dave Brand

American car speedos, in my recent experience, read true speed, as checked against GPS.

Wonder why the Americans use true speed then. But then again, America do have a funny way of doing things sometimes.

[JHamilton]

The speedometer in my truck is dead nuts. ¯\_(ツ)_/¯

[The Learner Driver]

Originally Posted by JHamilton

The speedometer in my truck is dead nuts. ¯\_(ツ)_/¯

Which truck do you have? Have you driven any others that are the same?