Speedometer accuracy/calibration
Senior Member
Joined: Nov 2011
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From: Everett, WA
GPS yields only a ballpark figure for speed. They are reasonably close on a ruler straight and completely flat road, but any deviation from straight and flat will give a low reading.
3-4 mph is a big difference however, so your speedometer may be slightly optomistic.
3-4 mph is a big difference however, so your speedometer may be slightly optomistic.
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Joined: Nov 2008
Posts: 2,606
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From: Calgary Alberta Canada
At what speed? In the absence of any practical way to make the speedo accurate it is deliberately made to read high. That way even the worst speedo will still read no more than actual speed. The usual tolerance is 3-5%. So, if your speedo reads 62 mph at an actual speed of 60 mph it is working correctly. I agree that reading 64 at an actual 60 is not acceptable but it would be to read 93 at 90 mph.
GPS speed readings are pretty accurate. The comment about the speed displayed by a GPS as reading low on bends leads to interesting philosophical questions about speed itself. The GPS remains accurate as it measures the actual change in position of the antenna over time but relative to spherical geometry. This raises the interesting question about what type of speed the posted limit relates to. After all, the speedometer technically only gives the speed of the wheel from which the speedometer takes its input. Usually that is the faster of the two drive wheels. So, on bends the speedo will over read actual vehicle speed by one half of the difference in drive wheel speeds. Which is a better indicator of actual vehicle speed? GPS which measures how rapidly you are approaching your destination or the speedo which measures how quickly your faster drive wheel is approaching your destination, which alternates for left and right hand bends.
Technically your car is travelling a range of speeds when traversing a bend, the left door mirror travels at a lower speed than the right door mirror in a left hand bend. How fast is your car travelling? If the average speed of all parts of your car is the correct speed then on average your speedo will read high in bends.
Finally, any official efforts to measure your speed will be pessimistic anyhow unless the officer stands directly in front of your car to measure its speed (or directly behind in which case an arrest is less likely). All other measurement attempts will be off axis and induce cosine error.
GPS speed readings are pretty accurate. The comment about the speed displayed by a GPS as reading low on bends leads to interesting philosophical questions about speed itself. The GPS remains accurate as it measures the actual change in position of the antenna over time but relative to spherical geometry. This raises the interesting question about what type of speed the posted limit relates to. After all, the speedometer technically only gives the speed of the wheel from which the speedometer takes its input. Usually that is the faster of the two drive wheels. So, on bends the speedo will over read actual vehicle speed by one half of the difference in drive wheel speeds. Which is a better indicator of actual vehicle speed? GPS which measures how rapidly you are approaching your destination or the speedo which measures how quickly your faster drive wheel is approaching your destination, which alternates for left and right hand bends.
Technically your car is travelling a range of speeds when traversing a bend, the left door mirror travels at a lower speed than the right door mirror in a left hand bend. How fast is your car travelling? If the average speed of all parts of your car is the correct speed then on average your speedo will read high in bends.
Finally, any official efforts to measure your speed will be pessimistic anyhow unless the officer stands directly in front of your car to measure its speed (or directly behind in which case an arrest is less likely). All other measurement attempts will be off axis and induce cosine error.
Last edited by jagular; Apr 30, 2012 at 03:22 PM.
Other factors to consider:
Have you changed the size of your wheels?
Have you measured the speed the "old fashioned" way, using a pre-known distance, cruise control, and a stopwatch?
Have you changed the size of your wheels?
Have you measured the speed the "old fashioned" way, using a pre-known distance, cruise control, and a stopwatch?
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Joined: Sep 2010
Posts: 251
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From: Austin, Texas
As per Jagular's comment, what speed were you going when you noticed the difference? Is it a constant 3 to 4 mph or is it a % of your road speed.
Speedometers often read 5% too high and I read somewhere that BMW speedometers are set to be always as much as 7% too high which seems a little excessive (I have checked the BMW we have and it is definitely higher than it should be but I have not had chance to accurately check calibration).
Notice that they always read higher speeds than the actual speed. That way you will not run into trouble with the law while trying to maintain speed limits.
If it is a fixed amount too high (ie. 3 to 4 mph at all speeds), then it may be a calibration issue and you need to have the car looked at.
Speedometers often read 5% too high and I read somewhere that BMW speedometers are set to be always as much as 7% too high which seems a little excessive (I have checked the BMW we have and it is definitely higher than it should be but I have not had chance to accurately check calibration).
Notice that they always read higher speeds than the actual speed. That way you will not run into trouble with the law while trying to maintain speed limits.
If it is a fixed amount too high (ie. 3 to 4 mph at all speeds), then it may be a calibration issue and you need to have the car looked at.
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Sorry, still don't get it. OBD stands for what? Where's the port?
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