Re: Speedometer Accuracy to 1/2% Accuracy - Here's How

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

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

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.

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

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

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.

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')

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

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 accuracy required.

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 guys.

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....

My Chrysler vans have very accurate speedometers. Both of them consistently read within 1 MPH of the local radar speed signs that the police like to set up in my area.

A tire makes a lot more than one revolution per mile! That is, of course, unless you are talking about a really large tire.

Matt

I've had absolutely no problems with the stone-age cable driven speedometers in my Trans Ams. The speedometer in my '71 T/A is still dead-on after 35+ years and 150,000 miles.

My 90 Reatta senses the rotation of the axle from a toothed wheel and a sensor. Then the digitial pulses are converted from pulses per unit time to a function of miles per hour and output to the digital dash. It's as accurate as it needs to be.

Tire wear wont make much inaccuracy...maybe 1-2 mph at 70 mph, or 2-3 % Under or over inflation might make a little difference, too.

But, honestly, I dont really care if it is off a skoshi.

I dont really see how Noman Nesciu's mouse model is going to work. If you're going to bounce a beam off the ground and try to correlate it to movement, you're screwed. You can bounce a beam off a wheel, or axle, etc which has a reflective digitizer on it, and you still have no better than what my 15 year old car has. You could use a GPS system if you had a lot of money you needed to pee away, or you could even chart the earths magnetic field and develop a global map which you could computerize and sense and find out about how fast you are going.

Does anybody really give a sheisse?

Simple doppler logic, easy to do. Chips for the speed discrimination - about 1.98

For crying in a bucket, it is a non-issue.

DAS

For direct contact replace nospam with schmetterling

I feel cheated now... that's 1800 miles of warrantee or in my case, nine days of business mileage.

Bill

Well, Im certainly no expert on this, but it wouldnt seem to be easy to do. And not needed anyway.

What are you going to Dopple...sound, radio waves, light?

You must go stand in the corner if you can't interact with the group positively...