Which is more accurate...speedo or sat-nav?

[spydoor]

Something I noticed the other day, wondered what you guys thought...

I've always assumed that speedos were set to read a little higher than the car is actually going.  Since getting a tomtom I noticed a discrepancy of over 5mph between the two.  Now, obviously if your speeding up/slowing down the sat-nav loses out because its working out your speed by doing the math on how far you've travelled in so much time.  But when cruising at a contstant 70mph (according to the speedo) the sat-nav reads only 65!
Just how accurate is sat-nav at working out your speed, if you're cruising?

[Ridg]

with 3 or more satellites the GPS should be more accurate, unless you've had your speedo recently calibrated.

the GPS will work out the speed between 2 points eg it works out the distance you've covered / the time to get from point a to b therefore there is a delay in the speed updating, however if you're driving at a constant speed and on a perfectly flat road then the GPS would have the real speed.

the speedo is always going to be inaccurate due to changing diameter of the wheel (due to wear, pressure, load etc) eg the distance you cover with each revolution (of the wheel) changes.  you've also got to take into account tolerances in the manufacture of the speedo, the size of the needle and the spacing of the dial / interval.

this is why police highway patrol car's have to be calibrated every so often, so that they can say you were travelling at that speed.

anyway i too have a tomtom (710) and mine also shows 65 when i'm doing (about) 70

[Daz...]

Sat-nav will be far closer to the real speed than your speedo. Although my speedo is accurate up to about 100 since I've changed the cable and drive gear on it. Helps that the gearbox hasn't done many miles either.

[Stu.750]

Ive got a speedo linked to somethin that measures distance on my taxi (you the meter)

 and when my speedo reads 35 im only actually doing 32 , and it get worse as i go faster

 So id say the sat nav would be closer

[jaybv6]

I think there is some kind of law that means the Speedos on cars over read but can never under read (with stock wheels fitted).  As a general rule mechanical speedos can over read by up to 10% and electronic speedo (not as in digital but the speed is calculated from the abs sensors rather than a cable) will over read by up to 5%.  Sat nav will be within 1% of the true speed.  So if you have a GPS verified speed it's more accurate.  That's why I never believe people that say "I've done 140mph in my car" as it's more like an indicated 140mph which could actually be 127mph (with the speedo being 10% over).  I only believe GPS verified speeds.