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Discussion Starter #1
I am just curious if anyone knows how accurate the Speedo is. It seems that I may be as much as 5mph under what it actually reads. I noticed this on the way back home from the dealer with my wife driving my other car. I also past one of those stand alone radar trailers (are those full of BS?) which showed me under about the same amount. Although this is not a big issue, it would be nice to know how much I am breaking the law!:)

Any experience with this?
 

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BrokenR1 said:
GPS's can vary a bit also. How far off are cars generally? My bike is around 6-7% which is normal.
If you hold a steady speed, or at least hover around a steady speed in a mostly-straight line, your GPS average should be within a fraction of a percent.

A GPS in straight-line steady motion for 60 seconds or so should be very very accurate. Imagine the start and end points being off by about 5 feet, and spread that error over the entire length of your timed run. It works much better than you might think.

Turning, acc/deceleration, and blocked view of the sky can monkey with your results, of course.
 

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I noticed on my roadtrip that the GPS and Speedo were quite a ways off in terms of MPH.
 

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Use Hwy markers and a watch with a seconds hand or better yet a stopwatch.

60 mph one mile per minute, so 5 miles in 5 minutes.

If that's too slow try going at 75 mph.

75mph = 1.25 miles (75/60) per minute so you can drive for 5 miles and it should take you just 4 mins.
 

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I noticed with the police speeding displays they are consistently reading about 5 mph less than what my speedometer says.
 

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ivan1 said:
I noticed with the police speeding displays they are consistently reading about 5 mph less than what my speedometer says.
Interesting. At slow speeds (say 18-20 mph) I've found the radar displays to match very closely (within .5 mph) with my bike's computer/speedometer thingy.
 

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I also find them to consistently be right on with my SUV....
 

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>>>I noticed with the police speeding displays they are consistently reading about 5 mph less than what my speedometer says.<<<

BMWs consistently read high..maybe they feel the market wants it.
 

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Stan said:
>>>I noticed with the police speeding displays they are consistently reading about 5 mph less than what my speedometer says.<<<

BMWs consistently read high..maybe they feel the market wants it.
I recall (but can't find) a Car and Driver article explaining why European cars tend to have speedometers that read high. It's a response to laws mandating that the speedo can never read lower than actual even if the owner puts different tires/wheels on the car. So they have to read high enough stock that any reasonable tire circumference change will still give a reading of at least the actual speed.

Interesting fact: My 2000 VFR speedo read about 7 percent high according to GPS but the odometer was dead on. My TT was the same way.
 

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I'm sure it is reading high. I have to get out the GPS to see just how much.
 

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Discussion Starter #14
Good suggestion on the GPS. I have one, but do not know why I did not think of it, must be getting senile....;) I would be interested in hearing various results. Eyelise, I heard that you were seen in Princeton recently. How many miles have you racked up?

Thanks for all the feedback...
 

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I'm glad that the speedo is high. That way there's a better chance I'll only get a ticket instead of cuffs when they finally catch me REALLY speeding. Of course I'm one of the goons who sets his watch five minutes ahead, too :rolleyes:
 

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Ground Loop said:
If you hold a steady speed, or at least hover around a steady speed in a mostly-straight line, your GPS average should be within a fraction of a percent.

A GPS in straight-line steady motion for 60 seconds or so should be very very accurate. Imagine the start and end points being off by about 5 feet, and spread that error over the entire length of your timed run. It works much better than you might think.

Turning, acc/deceleration, and blocked view of the sky can monkey with your results, of course.
But it's also interesting to see that when you have 3 or 4 GPS's together you can still get different readings on each one. My top speed on mine is saying 156 if I remember right.
 

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andykeck said:
I recall (but can't find) a Car and Driver article explaining why European cars tend to have speedometers that read high. It's a response to laws mandating that the speedo can never read lower than actual even if the owner puts different tires/wheels on the car. So they have to read high enough stock that any reasonable tire circumference change will still give a reading of at least the actual speed.

Interesting fact: My 2000 VFR speedo read about 7 percent high according to GPS but the odometer was dead on. My TT was the same way.
From http://www.caranddriver.com/article.asp?section_id=4&article_id=1906&page_number=1

Speedometer Scandal!
Can you trust your most frequently consulted gauge?
BY FRANK MARKUS
April 2002

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.


And here are the results of their testing: http://www.caranddriver.com/article.asp?section_id=4&article_id=1907&page_number=1
 
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