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Speedo vs satnav.

12K views 12 replies 10 participants last post by  M400WST 
#1 ·
Morning all,

Driving 1.4 corsa excite so hopefully somebody else with the same can confirm for me.

Speedo out by about 3-4 mph? I'll set cruise control to 70 and sat nav reads about 66-67

Or going through a 30 where it shows your speed on those signs. Sign will read about 27 when cruise is set at 30.

Thanks
 
#2 ·
Yes, mine is out by the same when compared with video footage on my dash cam (GPS set).

The law for car speedometers in the UK
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. Or to put it another way, if your speedo is reading 50mph, you won't be doing more than 50mph but it's possible you might actually only be travelling at 40mph.

To ensure that they comply with the law and make sure that their speedometers are never showing less than true speed under any foreseeable circumstances, car manufacturers will normally deliberately calibrate their speedos to read 'high' by a certain amount. As your satnav is not the designated device by which a car's speed is measured, it does not need to incorporate any fudge factoring.
https://www.thecarexpert.co.uk/how-accurate-is-a-car-speedometer/
 
#4 ·
Mine is the same. I do find if i'm on the motorway doing 73-74 according to car speedo (where satnav says it is 70mph), it really does hammer the fuel comsumption, so I generally stick to the car speedo and use the higher values to claw time back if I need to!
 
#5 ·
I really only ask because I don't want to be done for speeding,

I understand the whole 10% + 2 but sometimes for example on a bypass will set cruise st around 73 which then reads on satnav at 70
 
#9 ·
If you rely on the speedo you are safe by 10%.
If you rely on the satnav then part of your safety buffer is gone.
The remaining 10%+2 is an ACPO recommendation and not law. The new uk police commissioners can now overrule it if they get short of cash.
 
#11 ·
If the speed reading is higher say 33 when true speed is 30..is this effecting the true milage of the car?

I ask because i use to do a journey which was 205 miles door to door in my Insignia...
Once done it in a Civic and it was 4 miles longer and i can remember the Civics speed being way above the sat navs...
 
#13 ·
Unless your driving a Police Car which has a calibrated speedo with a calibration certificate, the chances are that you car will under read by 2-3 mph as that is the law.

That is also why they have the 10% + 2 buffer for the guidelines however a Police Office can user there discretion and do you for 31 in a 30 they don't have to follow the guidelines.

If you use the satnav in combination in cruise control so that it doesn't go over or use a speed limiter then you'll be fine, but for day to day driving just stick to the digital speedo if you have it.

The extra 2.5 mph in most circumstances isn't going to get you there any quicker
 
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