Low reading speedometers are costly in terms of police citations and create
safety problems as well.

Speedometers are inaccurate inherently and made worse as new tires are
installed and worn down due to the rolling radius variation.

A speedometer can be designed using the same principle as the optical
mouse. It reads the road and calculates V = D/T. Such a speedometer need
not be digital. A calibrated analog readout is feasible for 0.5% accuracy
at all speeds. It does not have to calibrate continuously if light or road
conditions are not within its capability. It simply uses memory and
reverts to the most recent self-calibration.

Some speedos are 5 or 6 mph off at road speeds. This should not be
tolerated in this day and age of high tech.

Such a speedo is quite elementary and involves absolutely no technology; I
have actually constructed a working model, breadboarded testbed. Although
it is the size of a shoebox and far from practical, it is entirely possible
to miniaturize it via a single dedicated I.C. I suggest this type
speedometer be standard equipment in the 2007 models and be made mandatory
by law.

Most speedometers, from the factory, read high. If it says you are doing
75, you are probably
doing less. Hence there is a bit of builtin safety. Cops in our state
normally wont ticket
you anyway until you exceed the limit by 5 mph.

Speedometers can often be calibrated by appropriate professionals. There
may be some
that cant be calibrated, but normally they can.

If you are worried about getting a ticket, run a calibration. Then dont
speed.

I sincerely doubt your optical mouse speedometer would maintain a high
accuracy over
varied types of road surfaces. There is no reason why it should.

On Sat, 26 Nov 2005, HLS (AT) nospam (DOT) nix wrote:

I don't think this is true on modern vehicles. It ceratinly used to be
true in the past.

I have checked my speedo in couple of ways:
1. Reading the speed indicated by a GPS unit and comparing it to the
speedo.
2. Reading the speed on roadside radar speed signs.

It's quite possible that the latter (roadside speed indicators put in
place by local police) may read high, to encourage people to slow down,
but I rather doubt that the GPS unit would be inaccurate. However, both
tests show my speedo to be as accurate as I can read it (ie. within 1
mph).

According to my GPS, my speedo is bang on. Also, as HLS mentioned, police in
Ontario usually don't touch you unless you are going 15km/h over except on
the 401 where you have to be going at least 40km/h faster than everyone else
before getting pulled over.

Here in NS, 10+km/h over will usually get you at least a warning, but I have
gotten away with more.

Steve

"Nomen Nescio" <nobody (AT) dizum (DOT) com> wrote

Yea, let's pass another useless law that costs us all money with no real
benefit. I've checked several vehicles against a GPS and found them all to
be pretty damn close. Close enough at least that if the owner gets a
speeding ticket it isn't the speedo's fault.
Bob

On Sat, 26 Nov 2005 01:10:03 +0100 (CET), Nomen Nescio
<nobody (AT) dizum (DOT) com> wrote:


Does Nomen Nescio by chance meen "Not a Clue"?????

Haven't seen ANYTHING of any substance from this poster in over a year
- just a lot of psuedo science babble nonsense.

"Nomen Nescio" <nobody (AT) dizum (DOT) com> wrote

We heard this from you about a month ago. Here we go again. Yawn.

Ted

Exactly. (Or they are correct.)

The OP is just polemical, based on an incorrect premise.

DAS
--
For direct contact replace nospam with schmetterling
---

<HLS (AT) nospam (DOT) nix> wrote

"Whoever" <nobody (AT) devnull (DOT) none> wrote

Good points. I cant say I own anything really 'modern' right now. My 97
Dodge van tracks pretty well with the radar signs, but those are normally
in 45-50 mph zones. I wouldnt expect too much offset there. My son
checked the van with his GPS unit, and said I was 5 mph slow at an indicated
70 mph. Could be. Or not..

Since speedometers actually measure a function of drive train rotation, the
effects of effective tire diameter, speedometer readability, accuracy and
reproducibility would be some of the factors that might cause you to be
off a bit.

Still, it is a miniscule problem in the grander scheme of things.

HLS (AT) nospam (DOT) nix wrote:

FWIW, the effects of tire wear would be less than 4% over the entire
tread life (based on 12mm wear depth and 26" effective tread OD).
That's about 2 mph at 60 mph. You'd like to think that the car and tire
manufacturers split the difference in their tolerance stack. IOW - they
should figure the nominal speedo reading at 1/2 tread life (and some
reasonable assumed tire pressure), so, everything else being dead nuts
on, the worst-case contribution to speedo/odo error from tread (OD)
differences would be 1 mph at 60 mph in either direction (1 mph low
reading with new tread, 1 mph high reading at end of tread life.

With a cog/pulse generated speed signal and exact (gear or chain and
sprocket) known final drive ratio, as you pointed out, other than driver
reading inaccuracy (paralax error, etc.), the tires (effective OD due to
tire size and pressure and vehicle loading), there should be no other
errors (OK - *very* insignificant parts-per-million error due to
computer timebase inaccuracy).

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')