As the speeding issue crops up again I thought I’d mention a little observation I made recently. I think most of us are aware or are under the impression that our speedos over read a bit hence why ACPO guidelines suggest officers can use their discretion to make some allowance for this. Recently I was plodding along some nondescript dual carriageway. On the dash was my TomTom. The speed on the TomTom was showing as 47mph. I looked at the Axor speedo and this was also reading 47mph. Previous posters have suggested the satnav speed would be more accurate than the speedo. I tried different speeds allowing time for any changes to be taken up. Every time the satnav matched the speedo exactly. So maybe we have to forget the habit of nicking a bit because the speedo over reads?
This depends on how well your tacho is calibrated.
Before mine was re-calibrated 3 weeks ago my speedo was reading 56mph but my satnav was saying 52mph 
I know that the truck was running slow as I couldn’t keep up with the other trucks on our fleet. Since re-calibration they both read 55mph 
Oh and I can now keep up 
I can’t see how you’ll get a bit of leeway for the speedo over reading at all.
Say you’re doing 50mph. If your speedo is over reading, it might be at 53mph.
So if you get stopped for speeding for doing 50 in a 40 limit, how can you use the defence ‘but my speedo said I was doing 53’ ?
It makes sense if they under-read, but none that’ve been through our workshops have ever under read the actual speed, they over read from the factory.
cieranc:
I can’t see how you’ll get a bit of leeway for the speedo over reading at all.Say you’re doing 50mph. If your speedo is over reading, it might be at 53mph.
So if you get stopped for speeding for doing 50 in a 40 limit, how can you use the defence ‘but my speedo said I was doing 53’ ?
It makes sense if they under-read, but none that’ve been through our workshops have ever under read the actual speed, they over read from the factory.
I don’t think you quite get it…I’ll give you some more time! 
There are legally specified accuracy figures for speedometers and tachographs.
Speedometers must not underread (i.e. show a lower speed than the true road speed), but are allowed to overread by up to 10%. Accordingly, manufacturers typically set them a few percent to the over side in order to ensure the speedo doesn’t underread even with worn and badly inflated tyres. Speedometers are often a bit non-linear as well - my car underreads far more at 60mph than at 40mph, for example.
This 10% tolerance in speedometers is the origin of the oft-quoted “10% + 2mph” tolerance for speeding offences. A car with a speedometer reading 55mph could be doing anything between 50 and 55mph according to where it is in the tolerance bracket. At 55mph on a speedometer, you should avoid a speeding ticket in a 50mph limit.
Tachographs should be set in a much tighter tolerance bracket because they are individually calibrated on the vehicle using a rolling road. In addition, the design of a tachograph should ensure it is much more linear than a speedometer - i.e. the error doesn’t change much across the usable speed bracket.
Having checked EU regulations, a tachograph must be accurate to plus or minus 6km/h - though most of the time it is going to be reading much less than 6km/h from the true road speed. At an indicated 88km/h (54.6 mph), the true road speed could be anywhere from 94km/h (58.4 mph) to 82km/h (50.9 mph).
As can be seen, driving an indicated 10% over the limit and assuming it’s OK in a vehicle with a tachograph is dangerous as you could be perilously close to a speeding ticket. In a car, you should always be going slower than your indicated speed. In a tachograph vehicle, you could be going faster than your indicated speed - at indicated 88km/h in a 50mph limit, you could be going 58.4mph, which is 1.4mph over “10% + 2 mph”. At slower speeds, the possibility of the tachograph reading 6km/h under the true road speed potentially puts you even more over “10% + 2 mph”. It is less likely that discretion will be used for commercial vehicles - they’re less likely to let you go if slightly over the limit than a car or motorcycle.
GPS indicated speeds are not necessarily accurate, especially on bends and hills, but they are typically more accurate than a speedometer. Many trunk roads have marked miles or half miles on them, not least to allow police to calibrate equipment such as VASCAR - if it is safe to do so, it can be illuminating to set cruise control (or drive on the limiter) and time yourself over the measured distance, comparing your calculated and indicated speeds.
Edit: Updated the tachograph section (highlighted in red) to reflect the +/- 6km/h speed tolerance in use figure from EUÂ regulations.
Our new MAN units are only about 1 mph out against my sat nav but my car shows about 3-4 mph less at about 50 mph.
This is why dimlow car drivers do 45 in average speed checks. Their dash says 50 
Got flashed by a speed camera, looked at the prat nav it said 48 and the NIP came through spot on 48 too. Speedo on the truck read 50 which is now confirmed as reading a tad fast.
When the truck is new the speedo will be spot on, as the drive axle tyres wear road speed will get lower, by the time they have been recut and near changing your real speed will be almost 2mph slower than satnav.
New set of boots and back to spot on.
Wife’s little Citroen C2 has the most accurate car speedo i’ve seen, numeral display on them, showing 70mph its really doing 69 on the sat nav.
My own car at 70 is doing 66.
My current Renault Premium shows 56 on the speedo but satnav jumps between 53 and 54. 
My old Stralis on the other hand showed 56 on the speedo but 58/59 on the satnav.
There is a red didgie speed indicactor attached to a post in Norwich, which I pass about 4 times a week, the speed on that is the same as on my Galaxy and it is 02 reg with 127,000 miles on the clock.
I’ve always figured it’s best to err on the side of safety. I have never come across a speedo that reads lower than the actual speed so if I stick to the indicated speed I know I’m ok - end of story. After 30+ years of driving I still hold a clean licence and so I reckon that my reasoning is ok - why try & work out if you can go 1 or 2 mph faster to beat the system?
i recently changed the back axle on my van, it must be a slightly higher ratio cos the speedo now reads slow, it takes a bit of getting used to reading that way!
I was in a brand new Transit van yesterday. Speedo was showing 70mph. Sat nav said 62!