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MINI2 Regular
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Discussion Starter #1
I've noticed from driving my old VW Polo (my Mini cooper is still in production), that my GPS system always tells me i'm driving approx 4 mph slower than my speedo does. so, in other words, my car is telling me that i'm driving 4mph faster than i really am.

I was just wondering for future reference whether or not this is somthing that occurs on the Mini Cooper as well, and if so by how much.

Thanks
 

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Astro
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mine does this also, ive got a Origin B2 and it always tells me im going slower,

i think car speads are in your favour, if you know what i mean!
 

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Track addict
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Minimatty04 said:
I've noticed from driving my old VW Polo (my Mini cooper is still in production), that my GPS system always tells me i'm driving approx 4 mph slower than my speedo does. so, in other words, my car is telling me that i'm driving 4mph faster than i really am.

I was just wondering for future reference whether or not this is somthing that occurs on the Mini Cooper as well, and if so by how much.

Thanks

Nearly all cars show a higher speedo reading than actual speed, and its usually a percentage of the speedos reading... usually around 5%. However, some speedos actually get less accurate the faster you go.... so at 120 its showing 130 etc.

My Cooper S Works shows a 4mph over reading at 100mph when measured by GPS... so in reality its actually doing 96mph - if GPS can be trusted as accurate - rememeber, GPS does not measure speed, it gives position. The device you are using calculates the speed from the changing position. Its only going to be as good as the software algorithms used. However, i would guess that its much more accurate than traditional speedos.

This inaccuracy gets exagerated at higher speeds. When my car shows 152mph on the clock :eek: I'm guessing its really doing about 143mph :rolleyes:

Obviously, this involved a recent trip to Germany's unrestricted autobahn, nothing to do with British roads at all!!!!
 

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MINI2 Regular
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Discussion Starter #5
yeah, i agree, its undoubtedly best to keep to the car speedo to avoid accidental speeding!

Thats good to know though, thanks!
 

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Mini Design Philosopher
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From my little knowledge; all cars should over read - and results from the fact when tyres loose tread, the diameter of the wheel decreases therefore a higher rpm of the wheel is required to maintain the same speed. I'm out of my depth of understanding here, but speedometers also need to compensate for the fact tyres increase in diameter at speed. Its dangerous for a car to under read.

If you look at full roadtests in the UK Autocar magazine, they always test the car at 30mph, 40mph, 50mph etc giving the true speed and the speedometer reading. Most cars are out by 1mph to 3mph at 30mph, and its quite common for cars to over read by 5mph or so at motorway speeds.
 

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Speedos can legally over read, but not under read.That's basically a green light for cheaper mass production of speedos - they don't have to be 100% accurate, so to avoid costs spiralling due to precise accuracy, they aren't! ;)
 

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I was very surprised with the accuracy of my Cooper's speedo. Especially at higher speeds :

GPS 75Mph = Cooper 77.5Mph
GPS 121.9Mph = Cooper 123,1Mph

But actually I read km/h's which were :

GPS 120km/h = Cooper 124km/h
GPS 195km/h = Cooper 197km/h

The GPS was a TomTom Go unit.
 

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as stated elsewhere all the speedos on MINIs are the same - no matter what size wheels you have. This makes a huge difference and they are probably much more accurate on a car with standard 15" wheels.

By the time you get to 16" or 17" that makes quite a gap for discrepancy.

Paul
 

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What's this button do?
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minisoopercooper said:
as stated elsewhere all the speedos on MINIs are the same - no matter what size wheels you have. This makes a huge difference and they are probably much more accurate on a car with standard 15" wheels.

By the time you get to 16" or 17" that makes quite a gap for discrepancy.

Paul
On another thread somewhere in the tyres forum, the 15s had the smallest overall diameter, IIRC, with the 17s next and the 16s being largest. If the 4mph error at speed were with the 15s, the error would be less with the 16s by about 2mph.
I wonder if the speedo was calibrated to the biggest size and then simply allowed to have increased error with the smaller diameter wheel/tyre combos...
-Eric
 

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I normally leave the speed warning of my SatNav/OBC set to 85mph, but it doesn't "ding" until the speedo reads 90mph. I wonder if the OBC is registering the true speed, with the speedo over over-reading by 5mph, which is probably typical.

The only other speed I've tried is 40mph, and for that the speedo matches the OBC.
 

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twouptons said:
On another thread somewhere in the tyres forum, the 15s had the smallest overall diameter, IIRC, with the 17s next and the 16s being largest. If the 4mph error at speed were with the 15s, the error would be less with the 16s by about 2mph.
I wonder if the speedo was calibrated to the biggest size and then simply allowed to have increased error with the smaller diameter wheel/tyre combos...
-Eric
Makes sense, I have 16''s and an error <= 2.5Mph.
 

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MINI2 Regular
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Discussion Starter #15
ahh, so then if i put, say (hypothetically speaking, of course) 20" wheels on a cooper, the speedo might start to under-read ever so slightly?
 

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Minimatty04 said:
ahh, so then if i put, say (hypothetically speaking, of course) 20" wheels on a cooper, the speedo might start to under-read ever so slightly?
It would depend on the tires. If you painted a small strip of rubber on the outside of the rim, you might still over-read :)

But yes, if your overall tire outside circumference got larger than what the speedo is expecting (which is about 24inches IIRC), you would under-read. It would also do bad things to your acceleration unless those 20-inchers don't weigh anything...

-Eric
 
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