Anybody know which is more accurate? The SatNav speed reading (Its a tomTom) or the car speedo? I'm guessing and hoping the satnav is more accurate (Had a speed gun pointed at me so trying to work out what my speed was so I can prepare myself for the dreaded NIP arriving )

SatNav without a doubt. You can test this yourself, find one of those smily face speed detectors normally outside schools etc and check the speed registered by TomTom and you car I'll guarantee the TomTom and the Smily will be the same. The speedo is often inaccurate up to +-10%. Mine is out 4mph at 30mph

The speedo on your car can be affected if you were to increase the size of your wheel,i.e cars standard alloy 13'' modified to 17/18'' as a bigger wheel will do less revolutions.Therefore it would read a slightly different speed reading.So i'd say that the speedos on cars are quite inaccurate.Whenever i drive i notice the speedo on the car says i am going 5mph faster than it says on the satnav.And as the police speedguns work via sattelite and so does the tom tom,then i would say its more a true reading off speed from the satnav.I think the speedos in cars make it out that your going a little bit faster than you actually are which can help you really,say if you go through a speed camera at 35mph you would have been speeding,but you were probably only really doing 30mph.Best investment is a snooper which is very accurate and even alerts you if they are hiding aswell

I think it's +10% -0%. They usually over-read for speed trap/safety reasons.

My sat-nav is ususally under the speedo.

For info, the ACPO guidelines allow for speed +10% + 2mph

So in a 40 that is 40 + 4 + 2 = 46.

We argued my son's 46mph in a 40 where they were going to charge. They dropped the case as we said that the allowance was as above therefore charge at 47. Also, we argued that in some areas you could do a speed awareness course rather than get points. In our area that was not available, so we offered he would go to Preston where he could. They said - No, so we said 'discrimination'

They said - forget it this time.
Result

David

I know a friend of mine was caught at 33 so 10% + 2 cant be an official line, this was by a chap with a gun and a NIP. 3 points and £60 IIRC.

There was a question about speedos a while back on Chris Evan's Fox the Fox slot on the radio, and in the answer, the Fox stated that it's a legal requirement in the UK that car speedometers must over-read the speed, and most do so by about 10%. So if your speedo says you're doing 30, you're actually doing about 27. Sat navs are not subject to this requirement so they give the actual speed.

The ACPO guide lines are as I stated. Some will charge at under those for the cash. Always challenge if within the guidelines. They will not want to take you to court and lose. 33 in a 30 is well worth the challenge.

I seem to remember reading somewhere that a typical speedo over reads from about 30mph upwards, getting more enthusiastic the higher the speed gets.
For example, a Kawasaki ZX9R speedo is around 20mph higher than a Garmin GPS at around 160 (GPS speed).

Ive tested my car against sat nav and I think the sat nav is much more accurate on a straight bit of road.

At lower speeds the speedo is pretty accurate (just a few MPH high) but at high speeds the speedo overreads my a large margin.
With the speedo reading 150 im only doing 138

That is unless you have an old police car thats calibrated
My tomtom and speedo match nicely