Speedometer Versus Satnav ...

[joko]

which is most correct?

i have noticed lately that my satnav always tells me im doing about 3-4mph less than the speedometer.

i would assume the satnav is correct as its digital and read an exact number, whereas the speedo is a pointer and so not as 'exact; in a quick glance.... bu then the speedo is part of the car...

so which is right?

i have watched this for a while so i am not misreading.

thanks

[baza]

I take the speedo reading over a sat nav but then I'm old.

[sunny-dave]

The satnav is right (although it will react more slowly to speed changes than the speedo).

Most car speedos are set to under-read by about 2 or 3 mph (my Mazda is a consistent 4mph out which is pretty poor) bu the satnav calculates from gps signals

[society]

Just keep a look-out for the coppers.

[EDDIE51]

This has come up several times before. Basically a 'speedo ' always reads slightly 'under' the true speed it is made that way to avoid giving an excuse for speeding. For example if a speedo read 70mph when you were really doing 75 you could argue that you were 'correct by the speedometer'.
So to avoid this if a speedo reads 70 mph the real speed will be about 67 mph.

[Old_Geezer]

I believe the sat nav will be most accurate as it doesn't have a tolerance to ensure it doesn't read low. However it will be slightly late since it needs to work the speed out from signals between it and the satellite.

[jno]

isn't that backwards, Eddie? The speedo overstates your speed, not understates it.

[SlackAlice]

Car speedometers are covered under the Construction and Use Regulations and must not under read a speed and need only to be within a range of 10% accurate.

Most of the cars I have driven have been about 5% accurate, although one was only 10% accurate.

Satnav generally works of off about 4-6 gps satelite signals and at constant speeds will be much more accurate. There is a slight lag in heavy acceleration/braking.

[EDDIE51]

jno as I said , if a speedo reads 70mph the true speed will be around 67 mph.
Speedos always over estimate the speed.

[Hopkirk]

The speedo will vary fractionally depending on the state of your tyres.

A new tyre has a greater circumference than a worn out one.