car speedo and sat nav discrepancy

usignuolo

I recently bought a "new" second hand car. I had it professionally inspected and it passed with flying colours and the dealer supplied it with a full year's MOT. Last week my OH and I took it to Yorkshire using his GPS.

He kept telling me to speed up when I was doing 70mph on the motorways until we realised the car speedo was showing about 70mph while the GPS on the dash, which he was looking at, was reading around 64mph at the same time.

He uses the GPS mainly in his car where it matches his speedo to within 1 mile. So I assume my speedo is out. How do I get it recalibrated?

phill99

Ho do you know it's your speedo that's incorrect?

robpw2

satelite navingation system software is not meant to be used to determine speed , its a guide

diesel_dog

Most car speedo's under read.

ariba10

diesel_dog wrote: »

Most car speedo's under read.

And by as much as 10%.

(I would be inclined to believe the sat nav)

reeac

robpw2 wrote: »

satelite navingation system software is not meant to be used to determine speed , its a guide

Where do you get this info. from?

Ultrasonic

SatNav's virtually always show a slower speed that car speedometers, though perhaps not by quite as much as you've found (but close, I think my satnav shows 65 or 66 when my car says 70). The discrepancy is less marked at slower speeds.

The usual consensus is that the satnav is likely to be more accurate, with car speedos deliberately set to read a little low. How far you'd get in a discussion with the police along those lines I don't know though...

robpw2

Ultrasonic wrote: »

SatNav's virtually always show a slower speed that car speedometers, though perhaps not by quite as much as you've found (but close, I think my satnav shows 65 or 66 when my car says 70). The discrepancy is less marked at slower speeds.

The usual consensus is that the satnav is likely to be more accurate, with car speedos deliberately set to read a little low. How far you'd get in a discussion with the police along those lines I don't know though...

sometimes your sat nav thinks your driving through a field when your on a road because the satellites are not quite aligned

mcfisco

robpw2 wrote: »

sometimes your sat nav thinks your driving through a field when your on a road because the satellites are not quite aligned

What do you mean they're not quite aligned? - they're in orbit and never stray from their path.
Sometimes you can lose a signal from a couple of them in poor terrain or because of tree cover etc. but you can usually see enough of them to get a very accurate position fix.
The mapping software is usually the culprit when it comes to visual errors on the screen.
I use a GPS on my bike for training and races and it's accurate to less than 1% - I'd take the GPS speed reading over a speedo every time.

kwmlondon

You may have the wrong size wheels on your car. The speedo could have been calibrated for 17" wheels but you've actually got 16" ones fitted.

Does anyone know if this would explain such a discrepancy?

I KNOW that my SatNav is bang-on as it consistently says my motorbike is showing 2mph slower than I'm really going. I KNOW this because the Gatso Camera agreed with the SatNav!

TonyMMM

Car speedos cannot under read - but are allowed legally to over read by up to 10%.

So if your car speedo says 70mph, the true speed can be anything from 63-70, so the SatNav maybe correct.