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Old November 19th, 2009, 05:34   #1
Gray06TDI
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Default Speedometer reading faster than I'm going

Hello everyone. I always keep my GPS on because I travel a lot. My GPS tells me my speed, and I think it's very accurate. On other cars I've driven, the GPS has either been dead-on, or off by 2 MPH or less. But in my Jetta, it's off by 4 MPH. So when the car says I'm driving 76 MPH, the GPS says 72. Has anyone else experienced this? Is there a way to fix the car's speedometer? Will this cause my odometer reading to be skewed as well?
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Old November 19th, 2009, 05:43   #2
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That has been the way the spedometer works in VWS for the last X years. X might be the last 10 years. Ever since they stopped using a cable attached to the transmission and the speedometer. Odometer is dead on with stock tires. Everyone who runs a gps in their car makes the same observation. No cure and it is not really a big deal. I know when my car says I am doing 75 in a 70 zone, I am safe unless I am driving in Ohio.
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Old November 19th, 2009, 05:47   #3
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What is the speedometer attached to? How does it know what speed to display?

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Old November 19th, 2009, 08:14   #4
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16" wheels instead of 17"?

I think it just reads the no. of revolutions of the wheel over time.
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Old November 19th, 2009, 08:21   #5
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Consider yourself lucky. My car has the opposite problem. When the speedo says I'm doing 72, the GPS says I'm doing about 76. This could lead to speeding tickets if I were not aware of the issue.

BTW: I have stock 16" wheels & tires.

Have Fun!

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Old November 19th, 2009, 08:57   #6
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The VSS (vehicle speed sensor) is in the tranny. There is a connector plugged to it. The info goes to the Cluster. I cannot remember if the info goes to the ECU first. However, since the cruise and other components need the info, it's most likely the speed data goes to the ECU first or may be sent back from the Cluster.

At 75 mph on the speedo, my GPS shows about 70 mph. The ScanGuage will show about 73 mph. The ScanGuage data comes from the same source that the speedo gets it's info.

When I buy new tires, I do several averages using mile posts ovr long distances and odometer accuracy checks (measured mile) to determine % over/under registering of my odometer. I put that data in the formulas of my Excel spreadsheet for calculating milage. Some tires will be slightly larger in circumference and under state milage while other tires will be slightly smaller and over state milage. Typically the adjustment is +/- 1%. Of course, as the tires begin to wear down, the factor changes.

As I remember, the older VWs (60s, 70s, & 80s) typically registered faster than actual speed.
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Old November 19th, 2009, 09:13   #7
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as a user of GPS devices for land survey applications, I can't believe a car mounted unit could be very accurate. It will give an excellent average over a long stretch, but readings over very short stretches would have a lot of "play". Also time of day and geography will influence the numbers by a lot.
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Old November 19th, 2009, 09:33   #8
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Quote:
Originally Posted by BobnOH
as a user of GPS devices for land survey applications, I can't believe a car mounted unit could be very accurate. It will give an excellent average over a long stretch, but readings over very short stretches would have a lot of "play". Also time of day and geography will influence the numbers by a lot.
I guess I should have been saying Garmin which is operated by GPS technology.

Also, I've noticed driving past the speed radar indicators beside the highway (DOT), that the Garmin will more often than not, be right on with them.
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Old November 19th, 2009, 09:48   #9
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Driven by a few of those road signs that say: You are going XX MPH. Every time I do I see it says I am going 4 MPH slower than my car says. Says 65 I look down and the Speedo says 69. So when I see a Police car I look at my speed and if it is with in 5 over I don't worry about it. I even pass State Patrol all the time while others stay behind him.
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Old November 19th, 2009, 09:50   #10
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My garmin is the same as the ScanGauge II which is 2 off all the time with the speedo. Stock tires. I learned the hard way with my 81's that the vws don't seem to always be accurate. I almost got a speeding ticket with the 81 vw but in the 04 Golfs case it is the opposite so if it says I am doing 57 I am doing 55 (81 vw says 52-51 and I am really doing 55 worse at higher speeds-75=82 and closer at town speeds-30=31 or 32
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Old November 19th, 2009, 10:33   #11
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This is likely due to European Union specification ECE 39, which requires that speedometers NEVER display a lower speed than actual, taking all tolerances into account. In other words, speedometers used to be designed to display the correct speed, with a ± tolerance (which in practice was almost always negligible). The new requirements are for the following:
0 <= (V1 - V2) <= 0.1V2 + 4 km/h
Where V1 is indicated speed and V2 is actual speed.

Incidentally, there is no such requirement for odometers, and odometers are frequently designed the "old way" to attempt to be as accurate as possible. My Jetta appears to function that way, with the odo dead on while the speedo shows high.
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Old November 19th, 2009, 10:39   #12
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Quote:
Originally Posted by JSWTDI09
Consider yourself lucky. My car has the opposite problem. When the speedo says I'm doing 72, the GPS says I'm doing about 76. This could lead to speeding tickets if I were not aware of the issue.

BTW: I have stock 16" wheels & tires.

Have Fun!

Don
Did you inadvertantly activate the "winter tires" feature in the MFD? It's supposed to correct for speedo differences. Maybe the assumption is that you'll put 15s on with snow tires.
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Old November 19th, 2009, 11:48   #13
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EU regulations require speedometer advance.

From a BMW motorcycles service bulletin:

Quote:
Speedometer advance is necessary to compensate for the negative tolerances in tire sizes between tire manufacturers, electric speedometer indication, and other factors. This advance reduces the risk of the speedometer displaying a speed that is less than the vehicle’s actual speed. The maximum amount of speedometer advance is 10% of the vehicle’s actual speed plus 2.4 mph.

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Old November 19th, 2009, 11:57   #14
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Quote:
Originally Posted by Gray06TDI
Will this cause my odometer reading to be skewed as well?
Gray, welcome to Fred's!

Quote:
Speedometer Scandal!!

Can you trust your most frequently consulted gauge?

BY FRANK MARKUS, April 2002


Pg: 1 | 2

Regular readers have probably noticed that when we describe a vehicle that really gets our juices flowing, we tend to hyperbolize about the accuracy and precision with which the steering wheel and pedals communicate exactly what is happening down where the rubber meets the road. It has recently come to our attention, however, that many of the cars we like best are surprisingly inaccurate about reporting the velocity with which the road is passing beneath the tires. Or, to put it another way, speedometers lie.

Yes, ladies and germs, we are scooping 20/20 and 60 Minutes with this scandal: Speedometers Lie! Okay, "exaggerate" may state it more aptly, if less provocatively.

When traveling at a true 70 mph, as indicated by our highly precise Datron optical fifth-wheel equipment, the average speedometer (based on more than 200 road-tested vehicles) reads 71.37 mph. Wait, wait! Before you roll your eyes and turn the page, let us dig just a bit deeper and reveal some dirt.
Sorted by price, luxury cars are the least accurate, and cars costing less than $20,000 are the most accurate. By category, sports cars indicate higher speeds than sedans or trucks. Cars built in Europe exaggerate more than Japanese cars, which in turn fib more than North American ones. And by manufacturer, GM's domestic products are the most accurate, and BMW's are the least accurate by far. One other trend: Only 13 of our 200 test speedos registered below true 70 mph, and only three of those were below 69 mph, while 90 vehicles indicated higher than 71 mph. Are our cars trying to keep us out of traffic court?
To understand, let's first study the speedometer. In the good old days, plastic gears in the transmission spun a cable that turned a magnet, which imparted a rotational force to a metal cup attached to the needle. A return spring countered this force. Worn gears, kinked or improperly lubed cables, tired springs, vibrations, and countless other variables could affect these mechanical units.
But today, nearly all speedometers are controlled electronically. Typically, they are driven by either the vehicle's wheel-speed sensors or, more commonly, by a "variable reluctance magnetic sensor" reading the speed of the passing teeth on a gear in the transmission. The sine-wave signal generated is converted to speed by a computer, and a stepper motor moves the needle with digital accuracy.
Variations in tire size and inflation levels are the sources of error these days. Normal wear and underinflation reduce the diameter of the tire, causing it to spin faster and produce an artificially high reading. From full tread depth to baldness, speeds can vary by up to about two percent, or 1.4 mph at 70 mph. Lowering tire pressure 5 psi, or carrying a heavy load on the drive axle, can result in about half that difference. Overinflation or oversize tires slow down the speedometer. All our speed measurements were made on cars with new stock tires correctly inflated, but one might expect a manufacturer to account for wear and to bias the speed a bit low; results suggest that not to be the case.
So we sought out the rule book to find out just how much accuracy is mandated. In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph.

But wait, there's more. Driving in arctic or desert climates? You're allowed another plus-or-minus two percent near the extremes of 20-to-130-degrees Fahrenheit, and yet another plus-or-minus one percent if the gauge was ever exposed to minus-40 to plus-185 F. Alternator acting up? Take another plus-or-minus one percent if the operating voltage strays two volts above or below the normal rating. Tire error is excluded from the above, and odometer accuracy is more tightly controlled to plus-or-minus four percent of actual mileage.
The European regulation, ECE-R 39, is more concise, stating essentially that the speed indicated must never be lower than the true speed or higher by more than one-tenth of true speed plus four kilometers per hour (79.5 mph at a true 70). Never low. Not even if somebody swaps a big set of 285/35R-18s for stock 255/45R-16s.
There's your explanation of high-reading European speedometers, with the highest readings on Porsches and BMWs that are most likely to lure owners inclined to fool with tire sizes. Of course, only the speedometer must conform. Trip computers are free to report average speed honestly. Try setting your BMW or Porsche cruise control and then resetting the average-speed function. Unless you've screwed up the tires, the trip computer should show a nearly accurate reading. Even General Motors, whose domestic speedometers are the best, must skew its readings slightly high on vehicles exported to Europe.
So there you have it: the raw, unvarnished truth about speedometers, laid bare without the underhanded aid of secret pyrotechnics. Readjust your comfortable indicated cruising speeds accordingly.
.
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Old November 19th, 2009, 14:21   #15
JSWTDI09
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Quote:
Originally Posted by MayorDJQ
Did you inadvertantly activate the "winter tires" feature in the MFD? It's supposed to correct for speedo differences. Maybe the assumption is that you'll put 15s on with snow tires.
First: No

Second: According to my manual, the "W" (Winter) selection in the MFD does not compensate for different size tires, but rather turns on a speed warning if you go too fast. This is because most snow tires do not have as high a speed rating as the regular (or Summer) tires. It makes no mention of speedometer accuracy.

Of course the manual has just as good of a chance of being wrong as I do, so your guess is as good as mine. There are several different threads here about speedometer accuracy and I am not the only one who has reported actual speed higher than what the speedometer says. VW's response is just that it is within "spec" and they have no interrest in trying to fix it. Different wheel sizes, tire pressures, and even different tire brands can effect speedometer accuracy somewhat. It would be impossible to build a car where the speedo is always correct, if you base the calculations on wheel rotation only (which is how almost all cars do it).

My speedo is not linear. At 40mph it is pretty much dead on accurate, however at 80mph it is high by almost 4 mph. If you know about how your speedo works, you can learn to live with it.

Have Fun!

Don
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