how accurate are the speed readings on these. Mine reads about 66 MPH when my speedo reads 70 MPH. My Tyres are about 1mm over the wear indicators so getting a bit low on tread, but would it make that much difference ? which should I believe ?

 

My S.E.C.S monitor says 66mph, when my speedo says 70mph
My tyres have got plenty of tread on them, i think alot of speedo's arent exact, just stick to the speedo speed and you wont get done

 

same as mine then. So to err on the side of caution (and a slightly slower journey) go with the standard speedo and not the GPS then.

 

My GPS and speedo don't coincide, when my speedo reads 30mph gps says 26mph
It's marginal, i tend to go with speedo just as it's easier to see

 

GPS systems are more accurrate as Speedo's tend to overread. Most manufacturers make them overread so that any discrepency is on the lower side of the posted speed limit. So if you stick to 70 on your speedo you can be sure that you are not over real limit.

GPS systems are more accurate due to their design (assuming someone wrote the software correctly). With the new EU global positioning going up soon it will offer even more accuracy that they are claiming may even be able to be used for vehicle control.

 

My Road Angel reads around 5mph lower than my speedo as well...

 

The EMP jamming system will not work at that range.
The Euro Sat. expense will be wasted on that bit.