A saddo discovery whilst trying to pass the time on the M6 on a long drive back home from Scotland.
If you set the cruise control at 70mph (according to the speedo), then re-set the trip computer, the mph showing on the computer shows as 66-67mph, which also accords with what my Navman satnav shows!
Anyone else noticed?

I brought this very thing up on another thread....

I recon the Average speed must use GPS to calculate.. Hi Darling, I'll be home in 10 mins. If I'm not back, please read this message again...

I've noticed this on a lot of modern cars who's dash and dials are done by elecrotrickery! For some reason despite the digital read outs showing more accurate numbers, the analogue dial will always show a more 'optimistic' view on speed. I think because most people expect around a 10% margin for error and therefore the manufacturers build this in so that we dont all get done for speeding! Freelander 2 TD4 Auto, SE, Narvik Black, Alpaca full leather, Rear Headrest Entertainment System, 6 CD, 18 HSE Wheels, Privicy tints

Not a question of expectation, it's the law. A speedo can read a certain percent 'over', but 0% under. (Sorry, don't have the exact 'over' figure).

I think where the expectation comes in is that it's generally accepted that before you are done for speeding, you are allowed a discrepancy of +10% +2mph against the posted speed limit. I.E. in a 70mph limit, you are generally not pulled if you are travelling under 70mph+10%, (i.e. 77mph), +2mph i.e. 79mph. Personally, I wouldn't push this, obviously.

It is also a well known fact, that using a GPS as a speedo, on a straight and clear road, will give a significantly more accurate reading than the fitted speedo in the car, for the reasons charrodine describes.

Make sure you check the GPS when you fit diferent sized tyres though  We Don't Care About Pot Holes !!

My previous car was the latest gen of the Honda Civic with a digital read out of the speedo and that too showed an optimistic view of the speed, i.e. I knew that if the digital speedo showed 74-75, I was doing about 70mph in reality.
This percentage 'over' actual speed also varied depending on how fast you were going i.e if my speedo showed 32 it was actually 30mph, if it showed 43 it was actually 40 etc
I think it is a safeguard for manufacturers from having people sue them if they are caught speeding.

Oh and agree that its better to check the GPS reading if you have different sized tyres than the original ones as the reading does differ slightly. Land Rover Freelander 2 GS Santorini Black, 2.2 Td4-e, Rear Spoiler