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BMW 3-Series (E90 E92) Forum > BMW E90/E92/E93 3-series General Forums > Regional Forums > UK > UK Off-Topic Discussions > Indicated vs. true car speeds.



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      07-16-2012, 11:50 AM   #1
Squintstream
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Indicated vs. true car speeds.

All,

A question for you...

We all know how the indicated speeds (on the speedometer) are not the actual speed in which the vehicle is travelling at.

For example, an indicated 54mph is approximately a true 50mph.

With this in mind, does it mean that a cars odometer is out as well? If I travelled at an indicated 54mph for 10,000 miles, it would actually mean that I had only travelled 9,200 miles (or thereabouts). Or does the odometer have something in place that works out the difference and reflects the true mileage

Not bothered, as most cars seem to do this. Was just interested to know

Cheers,


Charlie
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      07-16-2012, 11:57 AM   #2
dxb335d
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I'd say you are correct.

At Bruntingthorpe my 335d was only 3-4mph out at over 150mph
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      07-16-2012, 12:17 PM   #3
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I think BMW's have a 3mph offset (coming from my mate who's a tech in a BM garage)
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      07-16-2012, 12:25 PM   #4
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The speed is calculated by a wheel speed sensor. Depending on how fast your wheel is rotating your 'speed' can be all over the place. If you get oversized wheels and tires it will read low. If you get ones that are too small, it will read high.

It all has to do with the diameter of your wheel. Bigger wheels rotate less in a given distace, smaller wheels more. All you have to do to change how your speedometer reads is get bigger/smaller tires and wheels
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      07-16-2012, 01:04 PM   #5
wheelie good
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Yer last week when I got stopped by the cops for speeding doing 100 in a 30 mph

I smiled at the blonde PC and said he luv the reason you clocked me during a ton in a thirty zone was because of my new 20 inch new alloys

Went up from six to nine points for that, jeez no sense of humour
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      07-16-2012, 01:17 PM   #6
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Quote:
Originally Posted by Squintstream View Post
All,

A question for you...

We all know how the indicated speeds (on the speedometer) are not the actual speed in which the vehicle is travelling at.

For example, an indicated 54mph is approximately a true 50mph.

With this in mind, does it mean that a cars odometer is out as well? If I travelled at an indicated 54mph for 10,000 miles, it would actually mean that I had only travelled 9,200 miles (or thereabouts). Or does the odometer have something in place that works out the difference and reflects the true mileage

Not bothered, as most cars seem to do this. Was just interested to know

Cheers,


Charlie
Your speedo "error" will change from new tyres to worn tyres as the diameter/circumference will reduce with wear.
Comparing it with my SatNav, on my old E46, the speedo was absolutely correct with new tyres, but was reading about 5% out with the worn tyres.

Mike
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      07-16-2012, 03:52 PM   #7
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As far as I know it's illegal to have an under-reading speedo, therefore all manufacturers dial in a degree of over-read. My old E92 was under by 5% as is my current F10.
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      07-16-2012, 04:02 PM   #8
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the speedo's are designed to deliberately over-read.

your odometer is far more accurate (it needs to be, you cant have a car thats theoretically out by 10,000 miles over its lifetime because of speedo error). You'll also notice that if you set your cruise control to say 50mph, the dial will read 52-53. So the car knows what 50mph actually IS, speedo's are just typically calibrated for a bit of over read.

In the days of mechanical speedos it was usually a percentage of inaccuracy that got worse with speed, so maybe 3mph at 30, but 10mph+ at 100mph. current speedos run off of the cars electronic systems and are considerably more accurate and very closely calculated based on the cars factory specified wheel size. They also tend to be over by a specified and constant amount. E.g. always over by 2-3mph regardless of speed.

SO to answer simply, no, your odometer will not be out by anywhere near as much as your speedo. If you're running factory wheel sizes, it should be pretty much bang on.
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      07-16-2012, 04:30 PM   #9
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^ good post
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