Which speedometer???

[Darwinian]

Now, I've had this problem with my Lexus and my Supra and possibly my Saab and it's this..

When I'm travelling along at, say, 80 m.p.h. the speedometer on the dashboard will say 80 but the speedometer on my satnav will say 73. This happens with my Tomtom as well as my previous satnav.

Question is, which one is right? I would have thought that the calculations made by the satnav would have indicated an accurate m.p.h. but I'm not so sure.

Thoughts? :S

[LastPoet]

From the few mechanical knowledge I have the cars speedometer is innacurate (or was on the older cars, I dunno about the newer ones), and according to the law here they have a margin of error of 20 percent. Look here, the causes are several, including tire radius, tire pressure, etc.

I'd wager the satnav is more accurate, if you aren't rapidly accelerating, going in a straight line

[Darwinian]

Sounds about right. Motor companies not wanting to be sued by people caught speeding...

[theVOID]

*EDIT: beaten to it*

Speedos are usually calibrated to be faster than the actual speed because of the potential margin of error leading to drivers getting fined for speeding. If you see the same margin of difference on different cars and GPS systems this is likely the case.

What you should do is measure it at different speeds, 40mph 80mph 120mph - If the difference does not increase beyond the 7mph or so you are currently getting then I would be fairly certain that the GPS has the true speed and the speedo is clocked higher to account for this margin of error.

If the difference between GPS and speedo increases then you've probably got a problem or you're extremely unlucky

[leo-rcc]

It's a well known fact that speedometers in cars are not precise. Most measure the rotation of the main shaft, behind the gearbox, not the vehicle speed. So if you fit different diameter wheels your speed is going to be already off the mark. You can have your speedometer calibrated, but that is pretty costly. I find myself relying on the speed measurement of my satnav more than the one in my dashboard, specially if they are long trips. It's also pretty good at telling me what the maximum speed is of a particular road even if I missed the roadsign, which has saved me a few times as well.

[Tiberius]

What you should do is measure it at different speeds, 40mph 80mph 120mph

Good luck finding somewhere to go to 80 in the UK, let alone 120...

[theVOID]

You missed the word legally between 80 and in

[Darwinian]

What you should do is measure it at different speeds, 40mph 80mph 120mph

Good luck finding somewhere to go to 80 in the UK, let alone 120...

At 80 m.p.h. on the motorway I'm constantly being overtaken However, this makes more sense now if all the time I've been doing 74. 90 m.p.h. I'm usually keeping pace with the rest of the traffic.

[Welsh cake]

What you should do is measure it at different speeds, 40mph 80mph 120mph

Good luck finding somewhere to go to 80 in the UK, let alone 120...

The M25? That magical place where all those national speed limit signs are interpreted as national speed challenge signs.

[Oldandeasilyconfused]

In Australia the national speed limit is 110kmh. (about 68mph)


In my city there are no few than 8 permanent speed limits ,depending on where one happens to be .Between my house and the supermarket (about a mile) It's 50,70,60. The full range is: 40,50,60,70 80 kmh. 25 around schools and road works, 10 in car parks. 110 outside the metropolitan area.

Speed limits are enforced here. (hundreds of millions in revenue nation wide). We have red light cameras,speed cameras,and sneaky cops. On the Nullabor plain they have a spotter plane plane.I've been on a couple those European highways with no speed limits. They have some really spectacular accidents.