# Speedometer calibration



## Flyingman (Sep 13, 2009)

Runnerman2 said:


> Regarding swapping to a different size tire, thanks for the suggestions. But having just purchased a new set of "non-run-flats", that's not an option for at least another few thousand miles. I'll just live with knowing there's a difference in actual vs indicated speed. Thanks all!


Hey, compare the specs on your original tires vs the new ones, maybe they new ones have a larger diameter. It really can affect your speed/miles reading.


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## Flyingman (Sep 13, 2009)

TXPearl said:


> So some sort of average of the indicated speed of all 4 wheels?
> 
> That would help explain Flyingman's observation.


This is just too high tech for me! But explains why I'm seeing a slight closing of the gap between indicated and actual.

So why would speedometer and odometer read different? i.e. if I go down the highway at 60mph and the odometer says I actually did 63 miles in an hour.

My OBC is notorious for saying my mpg is around 27-28mpg but when I calculate my actual fuel consumed vs miles driven (odometer), I always get the fuelly value below, which is some 1-1.5mpg better.:dunno:

Regardless, after over two years and some 34k miles, my smiles/miles are still up there!:thumbup:


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## Penguin (Aug 31, 2003)

Flyingman said:


> if I go down the highway at 60mph and the odometer says I actually did 63 miles in an hour.


The easiest comparison which is accurate is to borrow someone's portable GPS unit and use it's speed measurement. GPS speeds are typically extremely accurate.


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## ProRail (May 31, 2006)

Flyingman said:


> This is just too high tech for me! But explains why I'm seeing a slight closing of the gap between indicated and actual.
> 
> *So why would speedometer and odometer read different?* i.e. if I go down the highway at 60mph and the odometer says I actually did 63 miles in an hour.
> 
> ...


Different systems.


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## Penguin (Aug 31, 2003)

Flyingman said:


> My OBC is notorious for saying my mpg is around 27-28mpg but when I calculate my actual fuel consumed vs miles driven (odometer), I always get the fuelly value below, which is some 1-1.5mpg better.:dunno:


Most MPG computers are not that accurate. While the fuel measurement is quite precise over small intervals for the purpose of controlling combustion, when the data is integrated over time to provide the volume of fuel used over a long period, a fair amount of error can creep in.

To a certain extend the ECM does not have to really know precisely the absolute volume of fuel it is injecting, as it simply uses feedback from the exhaust gases measurement to adjust the fuel trims to provide the right amount... a little bit like nudging the water faucet a little bit to increase or decrease the flow of water out of the garden hose without actually knowing the precise flow rate. If the "nudging" required to get the correct sensor readings gets to be too great, you get a MIL and an error code for fuel trim out of range or at the limit.


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## bigslickak (Jun 29, 2007)

My 335d is my second BMW, so after about 5+ years of driving these cars, my brain is kind of wired now to subtract 3 mph from what my speedo is showing. It's not that big of deal, now that you know, you'll get used to it and it will become second nature. The problem comes when you drive another car, and you find yourself going 78 mph instead of 75.


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## welds1 (Mar 10, 2017)

*Speedometer Difference*

I've noticed the same thing, at 73mph I,m actually doing 65mph. Im not concerned about the speed difference but I was wondering about the odometer. I'd like to know if we are ticking off more miles because of the difference in speed.


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## Michael47 (May 9, 2014)

Zombie thread from 2012.

For what it is worth, it is absolutely normal for German vehicles to read 3-4 miles an hour faster on the speedo, although the odo is bang on. This is due to a quirk in German law, I'm told. In any case, both the 1967 BMW 200CS and the 1969 BMW 2002 I owned back in the day, as well as a couple VW buses and our 2001 VW Jetta also do exactly the same. Our 2012 BMW X5 too.

So, to answer your question, @welds1, no, your odometer is not ticking off miles you are not traveling. The speedo is simply "German car fast," and completely normal.


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## welds1 (Mar 10, 2017)

Thanks for the reply, I checked the car against my gps yesterday and could see that the miles traveled were on the money. To be more accurate, the speedometer reads 73 mph and the gps reads 70.


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## Ozer (Sep 17, 2015)

I actually like it that my x5d is 3mph fast. When those Oh ****!! cop with a laser moments happen im actually going slower than what the dash is showing


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## Milliwatt Rob (Aug 27, 2016)

Actual speed is available at the OBD2 port and can be read with an appropriate device. I have a digital Scangauge 2 attached to the port, which I use to provide gauges that BMW left out. Mine is set to display, digitally, mph, voltage, engine water temperature, and instantaneous fuel economy.


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