Speedometer error vs odometer reading

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Something has been playing up in my mind recently. I used to own a car which over read the speed, when it was showing 120kmh on speedometer, the gps shows its actually 115kmh. My current car now is spot on with gps reading. So my question is which will give a lesser odometer reading than actual km / miles traveled. 1. Over Reporting speedo (speedo 120kmh actual speed 115kmh) 2. Under Reporting speedo (speedo 115kmh actual speed 120kmh) I would think its 2 ?
 
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Kyosho Optima Mid ...
Am I showing my age if I know what that is? I don't think you can make a hard fast rule about speedometer error and odometer error as with electronic speedometers the odometer could be correct when the speedometer was not. Odometer error can get a manufacturer in a little hot water (just ask Honda) but it think showing a higher speed than actual is not too uncommon...
 
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Cincinnati, OH, USA
Under reporting would cause the odo to be low. One of my best friends growing up had a 1st gen.(1984) Chrysler minivan in the family, they had the speedometer disconnected for a couple years so the warranty wouldn't expire on it. Now that was a cheapskate!
 

Kestas

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The last I checked, SAE standards allowed a speedometer error of -1% and +3%. I double check speedometers on the freeway using the mile markers and a stopwatch, and holding the speed steady at the speedometer. I run it for 10 miles to get a good average.
 
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Originally Posted by DuckRyder
I don't think you can make a hard fast rule about speedometer error and odometer error as with electronic speedometers the odometer could be correct when the speedometer was not. Odometer error can get a manufacturer in a little hot water (just ask Honda) but it think showing a higher speed than actual is not too uncommon...
This! Just because speedo is off doesn't mean that odo is off as well. Most speedos read high by design to prevent the consumer from sueing the manufacturer over a speeding ticket, especially if they end up using a slightly different tire size.
 
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As I drive past the visual signs that tell you what your speed is I glance at the HUD to verify it. They are always the same.
 

Kestas

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I once had a new car with a defective speedometer. I was able to beat a speeding ticket by showing the judge the speedometer was subsequently fixed under warranty. I omitted the fact that the defective speedometer read high!
 

alcyon

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Originally Posted by DuckRyder
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Kyosho Optima Mid ...
Am I showing my age if I know what that is?
oh yes it does ! The 1980s was probably the golden age of RC Cars and BMX bikes.
 
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Maryland USA
Originally Posted by alcyon
Something has been playing up in my mind recently. I used to own a car which over read the speed, when it was showing 120kmh on speedometer, the gps shows its actually 115kmh. My current car now is spot on with gps reading. So my question is which will give a lesser odometer reading than actual km / miles traveled. 1. Over Reporting speedo (speedo 120kmh actual speed 115kmh) 2. Under Reporting speedo (speedo 115kmh actual speed 120kmh) I would think its 2 ?
I was going to say #2, but I think the answer is not that simple. As pointed out earlier, the speedo could be off but the odo spot on. Take almost any car that calcs speed and odo from wheel rpm's: if you put tires on that are slightly larger, then your reported speed and odometer will read slightly less. There used to be some GM cars where the speedo was taken from the front wheel- if you put on a different size tire on the front, it would alter the speedo, but since it was a rear-drive car, the engine performance would remain the same. Another oddity- motorcycle speedo's used to be notoriously optimistic : 200 kph would actually be 180 kph- 5 to 10% error was not unusual. On boats and PWC (Seadoo, Waverunner etc.) they are called dreammeters, because they are at least 10% optimistic. Any serious mention of performance must note "gps or external measurement", otherwise it is ignored.
 
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Western S.C.
Originally Posted by Quattro Pete
Originally Posted by DuckRyder
I don't think you can make a hard fast rule about speedometer error and odometer error as with electronic speedometers the odometer could be correct when the speedometer was not. Odometer error can get a manufacturer in a little hot water (just ask Honda) but it think showing a higher speed than actual is not too uncommon...
... Most speedos read high by design ...
Both true, assuming OE tire size. Most recent odometers show under true distance, yet most speedometers show above actual speed. That inconsistency is deliberate. For many decades before Honda's legal problem to which DuckRyder alludes, most odometers indicated more than true distance.
 
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I am pretty sure on my truck the speedo is off and the odometer is pretty accurate. On the mazda the speedo and GPS are spot on. I don't know if the truck speedo is off due to age or just thats as accurate as its ever been. I assume an odometer would be reasonably accurate but still off due to tire wear.. My anal-ness on this subject has led me to never be able to change the stock tire size on my car.
 

Patman

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Originally Posted by Kestas
The last I checked, SAE standards allowed a speedometer error of -1% and +3%.
How does that explain why almost all BMW speedos are off by as much as a full 10%? (I've confirmed that my wife's BMW is that far off by all the road side radar signs I've driven past)
 
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Originally Posted by tiger862
And this is a argument for fuel mpg meters vs actual hand calculations
Not really, because a fuel mpg meter can be wrong, often by worse margins than a typical odometer. An odometer's systematic error is normally very consistent, and correcting for its error in your mpg calculations is easy. The standard cumulative mpg read-out in my car exaggerates by about 4.6%, much worse than the speedometer or odometer error.
 
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Originally Posted by tiger862
And this is a argument for fuel mpg meters vs actual hand calculations
Trust but verify. I have verified my mpg results on the Mazda and they are within a few tenths from each other. I consider that well within the margin of error.
 
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Waco, TX
My F150 is 5% slow (due to taller tires) For very 105 miles that I drive, my odometer only reads 100. When I do fuel mileage calculations, I can do them by the odometer (17.6 mpg) or by actual miles driven (17.9 mpg)
 
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It's closer today than in the past, but every OEM calibrates the speedo to read low with the OEM equipped tires at 0 tread life. If they are serious about being close, it will read properly when the tires are worn to the replacement diameter. But, today many vehicles use GPS for speed, so that will be almost right on. +3% sounds very tight, that must be a recent amendment or it's not correct. It definitely was closer to about +8% with older mechanical speedos (ie read 65, actual 60). It was clearly deliberate, as no speedo I'm aware of ever read low with the OEM make, model and size new tire. I seem to recall C4 Corvettes being almost right on and that was considered novel enough to be mentioned in reviews, but any OEM speedo will always read biased to high to account for tire wear.
 
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Regina, Saskatchewan, Canada
Originally Posted by alcyon
So my question is which will give a lesser odometer reading than actual km / miles traveled.
Here's a better puzzle for you. My G37's odometer matches highway odometer test strips, whereas my speedometer shows about 10 km/h faster than reality at highway speeds. If I plug in my Scangauge II, it's indicated speed matches radar testing, showing the lower speed than what's indicated on the speedometer. Hence, someone at Infiniti doesn't want owners screaming about speeding tickets, not to mention tire variation already addressed. The vehicle on some level certainly knows the correct speed, but won't report that to the driver unless prodded.
 
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Originally Posted by Patman
How does that explain why almost all BMW speedos are off by as much as a full 10%? (I've confirmed that my wife's BMW is that far off by all the road side radar signs I've driven past)
If the car was manufactured to European standards (not sure if it was), then 10% off is still within range, sadly. The speedo in my 530i is off by about 3 mph at most. At around 60 mph, that's about a 5% error. Speedometer regulations vary around the world. In Europe (UNECE standard), they are allowed to read 10% high + 4 km/h and still be "in compliance". Apparently, in the US, there is no official speedometer regulation laws for passenger cars, but most manufacturers follow a SAE standard. The below link has some more info, including what was already mentioned earlier that your odometer is held to a higher standard than your speedo: https://www.thrillist.com/cars/your...cy-in-german-american-and-japanese-cars#
 
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