Vehicle speed on GPS

When I used my old Garmin in various vehicles, the speed indication on the
GPS was about 5 mph lower than the vehicle speedo. Just got myself a new
tomtom 910, and this is the same. Is this a common problem with all GPS
systems, can anyone explain why.

TIA

wrote:


It's not the GPS it's the speedo - they are not allowed to read slow but
can read up to either 6% or 10% over (can't remember which). Sounds like
yours is particularly optimistic :-)

--
Jeff Gaines Damerham Hampshire UK
There are 10 types of people in the world, those who do binary and those
who don't.

A Ford then?

Heh, likely, mine over-reads by the maximum allowable.

This is a common problem with vehicle speedos. The GPS is the more accurate
figure.

Vehicle speedos are manufactured to deliberately over read slightly. Vehicle
speedos work by counting the number of times the driven wheels rotate. By
making the speedometer over read allowances can be built in to cope with
variations in tyre pressure and effects of centrifugal force which affects
the diameter of the tyres at speed. Thus no one can complain that they were
accused of exceeding the speed limit and driving at 35 mph because their
vehicle speedometer said they were only doing 30 mph. There is also some
slippage between the tyres and the road which also contributes to the over
reading. Spin the wheels on a patch of ice and your speedo will show 20 mph
plus, but the GPS will show that you are still only doing 2 mph (probably).

Mike