Faulty Speedo

[badlad.biz]

My friend borrowed a SatNav and tested it against his speedo. He found that his speedo registered 40Mph and the SatNav said 56Mph.
How accurate are SatNavs and is there any way his car could be lying to him.
Very Worrying, Speeding Tickets etc

[Miss Chief]

If the sat nav is correct, you're friend thinks he's 16 mph under the speed he's actually doing.

If the speedo is correct, no problem

So he can't get done for speeding??

[Disco Fever]

The Satnav will be out by around 10% as the signal has to travel up to the satalitte and back, so the when speedo says 50mph and the satnav will say about 44-46mph....

[Panic Button]

Absolute tosh Disco.

The Sat nav does not transmit anything.

[Panic Button]

The satnav will be more accurate than the speedo, so your friend needs to get his speedo checked.

One way to test if it is way off is to follow an HGV on a level stretch on the motorway. It will be doing 56mph, held at that by a speed limiter.

[TCL-MUMPING]

Speedometers are allowed to indicate a speed 10% higher than the true speed but must not show lower than the true speed. The Sat nav will be accurate so I would tell yir pal to get the speedo checked pronto.

[badlad.biz]

Can speedo's fail in this direction? I can see one failing total, but have never heard of one reading slower!
If the car had bigger wheel rims, I see it could error in this way, but would it make that much difference?
I checked my SatNav to my car by setting the cruse control to 60mph, and it says that i'm actually travelling slower than the speedo says.

[chas2008]

really need to know a bit more about the vehicle , ie, electronic or mechanical speedo ?

[BenDToy]

I recommend borrowing another sat nav, and check it that way. Sat navs are more accurate. By law speedos must not under read the actual speed. So if the speedo is "correct" he is probably travelling at about 36mph and the sat nav is over reading by 20mph. If your friend can't deide which is correct then I seriously question whether he should be driving at all! If he is driving at "30mph" in a 30mph limit is he racing past everyone else on the road? If yes then his peedo is probably faulty. If not then the sat nav probably is!

[north star]

Best check the speed by a passenger using a stop watch and the 100m marker posts by the side of the motorway. Much safer.

[DanAB]

or if you have an MOT/service coming up, mention it to the mechanic to see if there is a problem or not

[gsr600]

aint no help -
but i have yet to find an HGV that only does 56mph
can't be in the UK - some go at least 70!
i struggle to pass them legally :}

[rnchugg]

Gsr Contary to you comment HGV's in th Uk ARE limited to 56mph, BUT there are alot or irish and imported trucks with UK registrations that do in fact do more than 56. The other thing if the wind is in the right direction blowing hard enough with a slight gradient then it IS possible to get anything up to 70MPH out of one (not that I would know wink wink). Havin spent many years driving rigs it is possibly to "override" the limiter.
In response to badlad.biz original question. Your Sat Nav is correct. Get the car checked out.

[alavahalf]

Badlad...Have read through this thread and am now in a state on confusion...
Panic says ...follow a HGV to confirm your speed..
rnchugg... says we have trucks with changed registrations..
How will you know if you are following a british, irish or polish truck...

The answer is simple.....get your mate to sit in his car...
Put his ignition key in his pocket...
Pull the handbrake on firmly...
Turn the sat nav on...
If it says he is travelling at 16 mph...
His sat navs at fault...

Remove it from the car and bin it...!

[scrappylane]

every man n his dog knows sat navs are a false reading,they are way out,your speedos fine,if you put bigger wheels on your car or smaller wheels it affects your speedo

[Paul22118]

I go with the sat navigator, rarely inaccurate.

[badlad.biz]

Thanks Everone for your comments and suggestions,
Six of us including the Honda Prelude in question went off on convoy down the A14 at my 60mph, each with a SatNav: The results were amazing to me! It was obvious that the Prelude had a faulty speedo, but by how much was a puzzle.
2 Garmin SatNavs read the same speed, The other Tom Toms were all different, although not by much nore than a few mph.
My car (Kia Ceed) was 2mph lower than my Garmin, as was a Honda Civic. A Ford Focus and a Vauxhall Corsa were 1mph slower than a couple of Tom Toms. A ford Escort was 4mph slower using a Tom Tom.
The Prelude was registering just under 50mph her Tom Tom One said 59mph.
The Amazing thing was that all cars except the Prelude recorded 60mph. It was the SatNavs which were different.
Basically, we proved that the Prelude shouldn't be on the road and should be checked, although we can see why the driver has had two speeding convictions since he had the car. Maybe it should be part of the MOT.
One other thing, much to my irritation, Tom Toms seem more accurate satNavs.
Once again Thanks..

[Disco Fever]

Hey Panic, I have a Transit van that follows trucks everyday at 56mph and my satnav, a Tomtom always reads around the 50-52mph and my speedo always says 56mph so my Tomtom must be 10% out as it must get a signal from the satalites above, I must admit I was wrong with the transmitting thing....

[badlad.biz]

Disco Fever..
Forget the Lorrys!!
Are you saying your SatNav reads under your actual speed?
Scary to think a SatNav could give you a speeding fine!
I will have bad dreams about this, as I spend a lot of time blatting between Sleepy Suffolk, where they are very motorist unfriendly, and Tenby Wales, where they are even more motorist unfriendly.
Badlad.