Which do I believe?

[Kannon Fodda]

Vehicle speedos are inevitably wrong. They need to allow for the marginal changes in wheel diameter as your tyres wear so rotate fractionally faster, and if the tyre was under / overinflated that might be a difference too. But critically as in most cases they don't want you to go too fast (OK there are some places with a minimum speed), they want to read faster rather than slower.

Only vehicles with a guaranteed accurate speedo will be police traffic cars. Those have a special "home office" logo to show the calibration so the copper can honestly say the speed they are doing to chase you.

Not all car speedo`s are reliable...

When I got pulled on my Harley Davidson, I was doing 126 mph according to my speedo...

But the copper in his BMW said...... Do you realise Sir you were doing 115 MPH on a 40 limit road ...

That must have been so dissapointing

 

[Lenny HB]

All Speedos are digital these day it just the the indication is often given by a meter needle the same as a tacho.

 

[berni109]

Almost all speedos read over actual. It's in the vehicle makers interests to be stating better miles per gallon. Those 55000 miles on the speedo mean you have only travelled about 50K.
The old rule of thumb of posted speed limit plus 10% plus 2 miles/p/h have no basis in law. They can do you for 31 in a 30 if they are having a slow day.
It would seem from recent posts on here that the cameras on the continent are set at a lower threshold.

 

[Lenny HB]

Just to be perdantic the actual regs for Speedo accuracy are:-
-0 +10% + 10kph for the life of the vehicle.

English version of EU reg:

The amended Road Vehicles (Construction and Use) Regulations 1986 permits the use of speedometers that meet either the requirements of EC Council Directive 75/443 (as amended by Directive 97/39) or UNECE Regulation 39.[11]
The Motor Vehicles (Approval) Regulations 2001[12] permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.
For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.

 

[Lenny HB]

Those 55000 miles on the speedo mean you have only travelled about 50K.

In practice the odometer tends to be more accurate. I can only assume they are set with a lower tolerance also the Speedo inaccuracies vary with speed which would average out on a journey.

Checked mine against sat nav distance they were the same over 100 miles, although the satnav distance is relying on the mapping accuracy.

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[pappajohn]

The old rule of thumb of posted speed limit plus 10% plus 2 miles/p/h have no basis in law.

Agreed, but my example was based on most, if not all, forces adopting the 10% plus 2mph ideal.
As Lenny said, speedo error is entrenched in law.

The UK law is based on the EU standard, with some minor changes. A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph.

So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph.

 

[berni109]

@Lenny HB so hows that then? Engine rotation x gear ratio x assumed wheel circumference? Is there another way?
Then maybe you also need to take in map projections- wiki has a reasonable explanation.

@pappajohn agreed! But irrespective of what your vehicle speedo is reading if you are clocked, photographed or gunned at more than the posted limit you can face criminal charges. No if's no buts, if the law can prove the calibration of its equipment and they say you were exceeding the limit you are done.

 

[Bognor Saint]

Thanks for all the replies. From what I understand now it seems that I have not been going as fast as I thought so I need to speed up!

 

[pappajohn]

Another anomaly
You can never get from A to B faster than a satnag says it will take.

My last trip to York the nag said 1hr 20min.
I gained 8 mins but the arrival time adjusted back by 8 mins.

That's cheating. .

(1hr 20min is ridiculous for a 38 mile journey. 82% is 60mph, 15% is 70, 2% is 30 and 1% is stuck behind a truck/tractor)

 

[keithchesterfield2]

My Satnav hasn't a clue what speed I'm going.

She just sits there sucking on Mint Imperials and Werther's Original and then tells me I'm going the wrong way - I should never have married her over half a century ago.......

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[tonyidle]

satnav distance is relying on the mapping accuracy.

I don't know for certain but I'd assume that GPS times travel between two triangulated co-ordinates regardless of the picture (map) beneath them. If there's any inaccuracy in that timing then GPS positional information would be equally inaccurate.

 

[berni109]

GPS satellites see the globe in 3d and know within a few inches where everywhere is. A map is a representation of the globe on a flat surface. Inaccuracy over relatively short distances are negligible over larger distances they start to add up.
Travel time between a and b coordinates in a straight line at sea level can be accurately calculated. Twists and turns, up and downs and stops and starts together with speed of travel unless absolutely known will mean that an element of inaccuracy is introduced into the calculation.
Quite remarkable how close a satnav gets to the actual time taken to travel from a to b in the circumstances