What Speed am I doing?

Every Impreza I've driven has been off by about 4mph at 70mph according to SatNav & Android GPS at the same time (showing same results as SatNav).

My own Impreza which had larger wheels was out by a solid 10% all the way from 50 to 180.
 
I noticed when I was in America last year, the mustang and camaro I drove always matched with my tom tom speed. It's only UK cars that seem are off.
Don't know why.
 
Glaucus said:
Also aren't speedos calibrated for 30mph meaning further away you are from that speed, the less accurate they are. Always taken gps as most accurate.
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They are, at least to my knowledge calibrated so they never under read as a requirement of type approval in the UK.

As such, if the calibration is say +/- 3% your speedo will tend to read as a minimum 103% of your total speed in case your calibration is out that -3%.
 
Speedometers read "fast" to give the driver a cushion before he/she is speeding.
2mph, for example, is a small movement of the needle and it would be relatively easy to accidentally creep over the posted limit. By building in a +ve innaccuracy you'd need to be doing an indicated speed limit+5/6mph (or possibly more) to be actually travelling at 2mph above the posted limit and this would be deemed as a lack of attention by the driver.
 
On my torque pro app my speedo is +4mph to what it shows, since it's plugged directly into the OBD2 port I would imagine this to be fairly accurate. More than a satnav however, I'm not sure.
 
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