Speedometer testing?

Hello. I just bought a 530i 2003 to replace my totalled 540i. In doing so, I also bought a good Garmin GPS/NAV system. The Garmin calculates my speed
in mph, among other functions, to the nearest tenth of a mile.. I am noticing that the speed the Garmin calculates is about 5 mph below the stated speed on my speedometer when driving in the 70-80 mph range. In the 60 mph range, it is about 3-4 mph below the speedometer reading.
What is your experience with this? Is the GPS likely accurate? How is a speedometer tested?
Any suggestions? thanks, mark
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That's not completely unexpected: BMWs always read high, although rarely that high (there are some regulations that mandate higher-than-actual readings.) My '01 330xi reads around 2mph high at 60mph, and our '91 525i reads about the same.
I've checked the speedo using my GPS, radar warning signs along the road, timing, and comparisons with other cars. I've also asked Garmin, and they believe that their GPSs should be +- 0.5mph or better, IIRC.
FloydR
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In BMW's I have had (4) the speed warning device is 2-3% high and the speedo 5-7%. More recent cars should be better.
The easiest way to check is to time the car on cruise control at some set speed (e.g. 70mph) against the 100m marker posts at the side of the motorway.
Even the degree of wear on your tyres can affect the reading by up to 2%, however if the reading is out by more than about 8% check that the right wheels and tyres are fitted. I looked at a 750i recently, lovely 19" alloy wheels and 40 profile tyres - I rang a tyre dealer to check they weren't to big, only to find they were actually too small!
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On Wed, 1 Mar 2006 10:21:58 +0000 (UTC), "R. Mark Clayton"

Did it have lowered suspension, black tinted windows and gold emblems as well? I think I saw it too.
--
Dan.

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wrote:

No. It was otherwise fairly normal, just high milage, old and a bit tatty.
There is this velvet blue one on sale with prominent (oversize?) Alpina and 750iL badges all round (bonnet, front door, c pillar, boot (*3), piped leather etc. etc. The only problem is that under the bonnet it is a plain vanilla 740i.
See http://pictures.autotrader.co.uk/ATD_web/servlet/media?id '9886696 and http://pictures.autotrader.co.uk/ATD_web/servlet/media?id '9886683

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On Wed, 1 Mar 2006 15:42:06 +0000 (UTC), "R. Mark Clayton"

If it doesn't have the lowered suspension, tinted windows and gold emblems, then I'm not interested in it. A big-ass window-shaking stereo could tip the scales in its favour though...
There's a chap down the road from me with a tatty 840 with odd-sized and mismatched aftermarket wheels and a very tasteful spoiler on the trunk/boot lid that appeals to me more. He only wants $12K for it.
--
Dan.

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Dean Dark wrote:

you forgot the ";-)"
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-Fred W

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On Wed, 01 Mar 2006 21:34:44 -0500, Fred W

you forgot the ";-)"
I didn't forget. Honestly. That was a few months ago, he's probably dropped his price a bit by now. The windows were a dark tint, too.
--
Dan.

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wrote:

Well the height of nafness in the UK is to have a LHD car. I suppose that they are more common in the UK than RHD in the USA, but tend to be favoured by pimps and other show off criminals (serious criminals off course avoid ostentation).
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On Thu, 2 Mar 2006 11:27:54 +0000 (UTC), "R. Mark Clayton"

There are some nice cars that are well worth driving that never came in RHD... The Lancia Integrale comes to mind.
I always thought Smart cars were the height of naffness... Until I saw this!
http://www.jalopnik.com/cars/novelties/hoon-of-the-day-sportbikeretrofitted-smarts-157584.php
Now I want one!
Dodgy.
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MUSHROOMS ARE THE OPIATE OF THE MOOSES

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Original Audi Quattro (although it was blown).

http://www.jalopnik.com/cars/novelties/hoon-of-the-day-sportbikeretrofitted-smarts-157584.php
Later Smart cars are actually very nice for getting about town. Quite roomy (for two!), six gears etc., but getting expensive.
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Assuming you are allowing sufficient time at steady state speed for several GPS data updates (12 seconds or so) and are locked onto 3 or more satellites (usually you will be locked on 6 or more) the Garmin will be accurate to less than 1 mph - probably less than .5 mph or better. GPS is incredibly accurate measuring speed.
RCE
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waffled on about something:

I too have a Garmin, and as another poster has stated, they are very accurate.
My '96 E36 reads almost exactly the same as yours. 3mph high at 30mph, slowly rising to 5mph by about 60mph. At which point it stays 5mph high all the way to 100mph (which is as far as I tested to).
As for testing them, in the UK you're tested every time you go past the big yellow boxes on poles!
Dodgy.
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mdd wrote:

I don't think the "error" is built in since it is inconsistent. My X5 reads 1 MPH faster than the GPS at 60 mph. My 3301 reads 3 MPH faster at the same speed.
Since both are leases, I checked the car's odometer against the GPS. They are both very close but do run a bit higher than they should. Over 36 months/45K miles the difference isn't enough to cost me anything at the end of the leases for extra mileage. My guess is that, worst case, the 330i will pick up 200 miles or so over that time. I'm not that close for it to matter.
FWIW a Pacifica I rented last week was dead on at 80 mph with the cruise control set.
Dan
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Yes, the GPS should be accurate to a greater degree than the car.
If the car reports 70, you actual speed should be in the range of 67, give or take an mph. You are noticing a normal condition, but perhaps a bit on the extreme edge of what should be normal.
You can test with a stopwatch and a measured mile. The freeways have milemarkers on the side, and you can divide 3600 by the time it takes to go a mile with the Cruise Control set, and see if the speed that results matches the speed reported by the GPS or the speedometer.

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wrote:

This is normal. Seen it on every bmw. Your gps is accurate, the BMW is not. 5 mph sounds about right with what I have seen. Same story on Audi, I think they do it on purpose.
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