Modern speedometers appear to be very accurate - on Fords, at least. For the
last few years, Ford has offered only Hybrid Electronic Clusters in their
vehilce line. These speedometers are electronically driven.
It all starts with the VSS..... Up unitl about 05, Ford used a variable
reluctance sensor to generate the VSS signal. This is placed close to an
indicator ring that induces current flow through the sensor and wiring as
the teeth on the indicator ring pass the sensor. The module that gathers VSS
information (and shares it across the network) knows how many teeth the ring
has and uses the frequency of the signal to determine the speed of rotation.
After 05, the sensor is still a two wire sensor but generates a digital
rather than analog signal. With these new sensors, we should see fewer
concerns related to incoherent VSS signals.
After this, tire rollout is the only concern. Most new tires will have a
tread depth of about 12 to 15 32nds of an inch..... meaning that a new tire
is nearly an inch taller than the same tire when it is worn out. This
translates into about 3" per revolution.... What a horrible state of
affairs.... what? Thats' 3" in a mile? A mile is 63,360"?.... oh....I guess
3" isn't going to matter much....
One last consideration is what a tall driver sees. Theough the beauty of
parallax, a short driver will see the needle in a different position than a
tall driver will see. This doesn't mean much since a *smart* driver will be
travelling , in most instances, at the rate of traffic flow. There are times
I wouldn't consider this since most traffic pile ups occur when someone
suddnely tries to change the rate of flow. We read about this nearly every
year as fog banks and sudden snow storms catch folks unaware.
Living in western Canada, we use our speedometers to be sure we are going
slow enough to avoid a ticket (police give us a 10% fudge factor meaning I
can drive all day at 110 kph in a 100 zonewith few worries). We use our
odometers to determine our next service interval... no great degree of
If I'm driving to Edmonton ffrom Slave Lake, I know that it is about 2 and a
half hours in most weather... it doesn't matter how far something is, it
only matters how much time it will take to get there.
Living in the Great Arboreal Forest, our country side is crosshatched with
logging roads and oil patch roads rangiung from winter travel only to
fantastic. Even here, there is no great need for inch for inch speedo
accuracy.... "Its the second left after the old Camp 8 road" works for most
To sum it all up... If I were to kvetch about an inaccurate guage..... I'd
likely pick either the oil pressure of fuel level guage as my pet peeve....