Speedometer calibration
#1
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Just in case anyone is interested, I compared my car's speedometer to the readout on a Tom Tom GPS. My car's needle actually sits a smidge higher than the digital speedometer, so I'm using the digital speedo and cruise control here.
The speedo reads 2-3 mph higher than the GPS at highway speeds. That is:
Car 75 mph. GPS 72 mph.
Car 70 mph. GPS 68 mph.
My car is a 2012 A6 2.0T with P+ 18" wheels and near new Continental ContiProContact 245/45R18 tires.
So I'm pretty comfortable aiming to keep the needle in the +5 range over the speed limit.
The speedo reads 2-3 mph higher than the GPS at highway speeds. That is:
Car 75 mph. GPS 72 mph.
Car 70 mph. GPS 68 mph.
My car is a 2012 A6 2.0T with P+ 18" wheels and near new Continental ContiProContact 245/45R18 tires.
So I'm pretty comfortable aiming to keep the needle in the +5 range over the speed limit.
#2
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Basically, for a larger tire, since there is more surface area and a larger circumference on the outside edge of the tire, the wheels will actually spin slower at any given speed than the stock setup would. Therefore, the speedometer in the vehicle will read less than the actual speed that is being traveled. Likewise, a smaller tire on a low-rider, for instance, will have to spin faster than a stock one to maintain the same speed, and therefore will cause the speedometer to read faster than the actual rate traveled.
You would think that they could use the GPS speed of the built in GPS unit to get 100% accurate speeds, but if the GPS ever failed (tunnels, mountainous areas, high-rise building, satellite issues) that would not be such a good idea, so they still use the "old tech" to determine the speed ...
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This is the case for every German car I have ever owned or driven - they all read a few mph faster than you are going with standard size tires. I believe it's due to laws in Germany not allowing the speedometer to show a lower speed than the car is actually going, even when using non standard tire sizes.
Basically, for a larger tire, since there is more surface area and a larger circumference on the outside edge of the tire, the wheels will actually spin slower at any given speed than the stock setup would. Therefore, the speedometer in the vehicle will read less than the actual speed that is being traveled. Likewise, a smaller tire on a low-rider, for instance, will have to spin faster than a stock one to maintain the same speed, and therefore will cause the speedometer to read faster than the actual rate traveled.
You would think that they could use the GPS speed of the built in GPS unit to get 100% accurate speeds, but if the GPS ever failed (tunnels, mountainous areas, high-rise building, satellite issues) that would not be such a good idea, so they still use the "old tech" to determine the speed ...
Basically, for a larger tire, since there is more surface area and a larger circumference on the outside edge of the tire, the wheels will actually spin slower at any given speed than the stock setup would. Therefore, the speedometer in the vehicle will read less than the actual speed that is being traveled. Likewise, a smaller tire on a low-rider, for instance, will have to spin faster than a stock one to maintain the same speed, and therefore will cause the speedometer to read faster than the actual rate traveled.
You would think that they could use the GPS speed of the built in GPS unit to get 100% accurate speeds, but if the GPS ever failed (tunnels, mountainous areas, high-rise building, satellite issues) that would not be such a good idea, so they still use the "old tech" to determine the speed ...
#4
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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.
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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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#7
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I haven't measured it on my A6 yet, but on my two previous cars, a Lexus and a Toyota, the speedo would also read higher than actual, but the odometer and tripmeter would read about 1% less than the tripmeter in the GPS, mile markers and Google Maps. So it is separate from the speedo and don't worry because it's in your favor.
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#8
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My experience with all of my past and present Audis has been different - both speedometer and odometer read about 1-1.5% higher than actual. The two seem to be tied together..
I've tested speed against two different stand alone GPS units and mileage readings against several test series of mile markers along the interstate. Try it and see what percentage of difference you may have.
I've tested speed against two different stand alone GPS units and mileage readings against several test series of mile markers along the interstate. Try it and see what percentage of difference you may have.
#9
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My experience with all of my past and present Audis has been different - both speedometer and odometer read about 1-1.5% higher than actual. The two seem to be tied together..
I've tested speed against two different stand alone GPS units and mileage readings against several test series of mile markers along the interstate. Try it and see what percentage of difference you may have.
I've tested speed against two different stand alone GPS units and mileage readings against several test series of mile markers along the interstate. Try it and see what percentage of difference you may have.
#10
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Basically, for a larger tire, since there is more surface area and a larger circumference on the outside edge of the tire, the wheels will actually spin slower at any given speed than the stock setup would.
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