Gilesdwrc

Since I bought a garmin I've noticed that either my spedo over reads or the satnav under reads. so which is it? how accurate are sat navs for speed? I know that by law car spedos have to over register but by how much?

satnav > spedo
30 > 33-34

50 > 55

70 > 75-76ish

90 > Don't know, not allowed to go that fast, but I'm assuming it might be 97ish.

El Cat

sat navs measure GPS from global p[ositioning satellites
ur speado is gonne be wrong, even newer cars are a little out
a speedo will (should) always over read as otherwise you will be speeding if it underreads

what would scooby do

satnav wins

speedos have to over read by about 10% to stop people suing the makers on speeding offences.

scoobynutta555

Satnav. All my cars have read 5 mph slower on the speedo at 70 mph.

T1000

Sat Navs are very accurate but they take a several milliseconds to refresh the speed all the time.

When I used to live in Germany, I thought my BMW 318i had a high top speed of 140mph on the speedo. The Sat Nav said it was 126mph though :-)

El Cat

mate of mine had a a VOLVO 850 T5 running 318BHP(proven) he went to germany with and whilst on the autobahn he opened it up
speedo was way past the 150 mph on the clock, said it was the equivalent of where 175mph would roughly be, sat nav said 157mph GPS speed!

Mark'sWRX

Sat Nav is accurate. I've tested it in a cop car with a calibrated speedo and it was spot on. The Scoob speedo is way out, but so are many 'quality' cars. The only one I've found to be correct so far is a LR Disco3!

GC8

Your speedometer will be far more consistent. Where the satnav is reading a continuous speed vs. your speedometer, then it will be more accurate.

AndyC_772

iTrader: (2)

Speedo is required by law to have a tolerance of -0 / +10%. It may well be consistent, but in absolute terms, not all that accurate.

The most accurate speedo I've come across was the one in my '05 JDM STI, which of course started out life reading in km/h and was converted to mph as part of the SVA process. My current '95 STI is pretty close too.

The Greenwood

Sat-Nav is by far more accurate, just remember that as the Nav gathers its triangulation information from sats navigating the earth from above it fails to notice any hills or steep inclines, hence slight miscalculations can occur.

SwissTony

iTrader: (19)

I always thought that my satnav was more accurate than the speedo.
(especially my small speedo trunks )

This is made worse when you take into account the rolling radius of the wheel, the larger you go up on the classics, the further out your speedo is.

Stainy

Now work out your MPG again based on the TRUE reading

scooby L

The Porsche speedo was only 2mph fast compared to the gps.

BMWhere?

Your speedo will read differently if you change wheels size and also you'll get a variation between new and worn tyres.

GPS will give an accurate speed, but very much depends on signal quality. Also the refresh rate of most GPS displays is quite slow, maybe only once/second, so the GPS speed is only relevant when you drive at a steady speed.

amahrap

By a strange quirk of fate my speedo is 100% accurate to the sat nav - new age, 19" wheels, 35 profile tyres

i thought that the wheels would mess it up but in my case they have actually made it better !

MarJay

I used to think that too, but apparently modern GPS's don't use your position to measure speed, they use doppler shift which is really really accurate. According to people in the know anyway.

Ciaran

iTrader: (16)

my bloody garmin stores your top speed!!!
The bird looked at it one day and it said !42 she was not impressed!!! bloody thing have to reset it all the time now

Marky9074

A modern receiver uses both the change in position, and the 'apparent' doppler (the latter being more important) in it's least squares computation and kalman filtering.....

Removed.

I've done a comparison between a Garmin, and a TomTom, in my previous Scooby and my mates Toyota, which had built in satnav. The Sat navs were all within 1 mph of each other, and both cars consistently read high. I think Sat Nav's are fairly accurate. My TomTom also agreed with the Gendarme that suggested I slow down after zapping me with his speed gun, then telling me what a cool car I had and letting me off with a warning as it wasn't worth the paperwork to fine me as I was 1 KPH into the 1st fine "band". Not sure they'd be so kind now.

Chelspeed

As said by law speedos must be -0%, +10%. This law was written in 1842 (or so) when speedos were twisty magnets and hair springs, +/-5% was as good as they could hope to achieve so the powers that be quite sensbily said -0%, +10% so that nobody could speed by mistake.

Nowadays with electonic speedos they could be +/-0.1%. So have the rules tightened up? Have they buggery.

So the thieving car makers set the speedos up to +9.8%, +10% because they can and because that makes their cars seem to go 10% faster which makes us all well endowed for the bragging market.

Well it did until we all get sat navs.....

My car, MY04 STi on 18", has also experienced the fit new bigger wheels and it ends up virtually spot on too.

Ant

my classic wrx is exactly the same is that down to the kph to mph on the speed cable?