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I have a Garmin Nuvi that shows your speed. Oddly, it is exactly and consistently 2 mph slower than what my Audi Q3 speedometer is showing.
This leads to all kinds of questions around your real speed and mpg, etc.
By the way, I also tested the same Nuvi in my husbands BMW 335i and it was spot on.
Any thoughts?
 

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I'd bet that anyone would tell you that 2 mph at somewhere near 55-65 mph is within +- 1% which is where anyone could/should expect a speedometer to be accurate ...
 

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Yea I think that is still pretty accurate. It could be that the Garmin is wrong and your car is right too.

Sometimes I think the manufacturer may purposely make it say you are going 2 mph faster than you actually are to stop you from speeding or something like that.
 

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Yea I think that is still pretty accurate. It could be that the Garmin is wrong and your car is right too.

Sometimes I think the manufacturer may purposely make it say you are going 2 mph faster than you actually are to stop you from speeding or something like that.
That would be a way to save themselves from you blaming them for reporting a slower speed which resulted in you getting pulled over and getting a ticket.
 

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They do the same thing with fuel gauges. That is why empty isn't actually empty. You can still drive for a while longer.
 

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They do the same thing with fuel gauges. That is why empty isn't actually empty. You can still drive for a while longer.
I've experimented with that with a few vehicles of mine and rentals and i end up being able to go 5-10 miles more than claimed.

But getting that low in fuel and often isn't good for your fuel pump.
 

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I've experimented with that with a few vehicles of mine and rentals and i end up being able to go 5-10 miles more than claimed.

But getting that low in fuel and often isn't good for your fuel pump.
I agree. Not good to push it because you could run out of fuel and because its bad for your car. But how many times have you really been pushing to get to the gas station basically on empty. Imagine if empty literally meant empty.
 

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Speedometer deviation explained

The speedometer is not a precise instrument. Precise instruments only work under laboratory conditions (steady temperature, humidity, environmental air pressure etc.).
Therefore the (European Community) car regulations foresee following regulation / allowable deviation for speedometers :

  • The speedometer may NEVER indicate a speed lower than the actual driven speed.
  • The speedometer MAY indicate a speed equal to the actual driven speed or a higher speed within this limit : ‘actual speed + 10% + 4 km/h’.
Example : when you drive 100km/h your speedometer CAN indicate a speed in a range from 100 km/h up to 114km/h (100 +10% + 4 km/h).
So far the theoretical approach.

Normally seen the car manufacturers order their suppliers to deliver speedometers with the smallest possible deviation, but as I wrote before the speedometer is not a precise instrument !

So if you drive 2 identical cars it is most possible that they indicate a different speed when the actual driven speed is the same…

I hope this explanation helps to clarify this phenomenon.

Best regards from Belgium / Europe ;)
 

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Thanks for that reference : here are a few more :

Reference

Current US FMVSS - Note where FMVSS 127 has been rescinded and is now merely empty but reserved

Current US FMCSA - Note, while this does NOT refer to cars, it's safety implication has similar merits that the FMVSS no longer addresses - interesting...
 

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The speedometer is not a precise instrument. Precise instruments only work under laboratory conditions (steady temperature, humidity, environmental air pressure etc.).
Therefore the (European Community) car regulations foresee following regulation / allowable deviation for speedometers :

  • The speedometer may NEVER indicate a speed lower than the actual driven speed.
  • The speedometer MAY indicate a speed equal to the actual driven speed or a higher speed within this limit : ‘actual speed + 10% + 4 km/h’.
Example : when you drive 100km/h your speedometer CAN indicate a speed in a range from 100 km/h up to 114km/h (100 +10% + 4 km/h).
So far the theoretical approach.

Normally seen the car manufacturers order their suppliers to deliver speedometers with the smallest possible deviation, but as I wrote before the speedometer is not a precise instrument !

So if you drive 2 identical cars it is most possible that they indicate a different speed when the actual driven speed is the same…

I hope this explanation helps to clarify this phenomenon.

Best regards from Belgium / Europe ;)
This is good to know. I had never looked up the exact regulations, I just guessed based on what makes sense to me. I wonder if you take it in if they can adjust it or something?
 
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