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Discussion Starter #1 (Edited)
2015 3i Sport hatch AT (1k miles)

I set cruise control to 62mph.
my garmin gps says im going 63mph.

my garmin has matched mph for 3 other cars I've used it in. (all cruise control.)
so I have confidence my garmin is reporting the correct speed and my new 3i is 1mph faster than what cruise control is set at.

WHY?
and is there an easy fix besides taking it to the dealer? (not worth the hassle)
 

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Speed via GPS is at best an estimate. It's just measuring the distance and time between pings. Your car measures the rotation of its wheels on the pavement, and by nature is more accurate.

With that being said, it's possible that the cruise control will fluctuate within a couple mph from the set speed as you go up and down hills, especially if it is the vanilla cruise control rather than the radar cruise control (radar cruise control can operate the brakes).
 

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Admiral Obvious
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1 mph, woop de do

US DOT allows a manufacturer up to 5% variations in speedo accuracy.

So at your speed it is within the allowable specifications, (60 +/- 1.5 mph)
 
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1 mph off is nothing to worry about. Just enjoy the car already.
 

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Speed via GPS is at best an estimate. It's just measuring the distance and time between pings. Your car measures the rotation of its wheels on the pavement, and by nature is more accurate.
No, that makes it *less* accurate, because the car doesn't know what the exact diameter of the tires is. The diameter of tires varies a lot with both air pressure and tread wear. Not only that, many cars (including this one) have multiple options for wheels/tires (16" vs 18"), so they program the speedo for one value, and while they pick combos to get a close match on overall diameter, it's usually 1% or so off, which is enough to see a speedo difference at highway speeds.

GPS speed can be very accurate (compared to speedometers on cars at least), but it depends on the GPS device maker and how they implemented it. If you're trying to compare your speedo to GPS speed using cruise control on a flat road, the fact that GPS averages it a bit isn't a bad thing, in fact more averaging would be better.

Also, I have the radar cruise control and it isn't all that accurate either on hills, and this is probably by design. It's better to save fuel by being a little "lazy" on speed accuracy rather than keeping it at exactly the set speed. It can operate the brakes, but it only does so when you have a good reason to slow down (like it detects a much-slower car in front).
 

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No, that makes it *less* accurate, because the car doesn't know what the exact diameter of the tires is. The diameter of tires varies a lot with both air pressure and tread wear. Not only that, many cars (including this one) have multiple options for wheels/tires (16" vs 18"), so they program the speedo for one value, and while they pick combos to get a close match on overall diameter, it's usually 1% or so off, which is enough to see a speedo difference at highway speeds.

GPS speed can be very accurate (compared to speedometers on cars at least), but it depends on the GPS device maker and how they implemented it. If you're trying to compare your speedo to GPS speed using cruise control on a flat road, the fact that GPS averages it a bit isn't a bad thing, in fact more averaging would be better.

Also, I have the radar cruise control and it isn't all that accurate either on hills, and this is probably by design. It's better to save fuel by being a little "lazy" on speed accuracy rather than keeping it at exactly the set speed. It can operate the brakes, but it only does so when you have a good reason to slow down (like it detects a much-slower car in front).
Fair enough. I guess I was trying to get at the point that it's not an instantaneous speed, and various factors (inaccurate clock, interference, etc.) could cause issues in relying on it as being an accurate reading.

And I agree that the radar cruise control is lazy. In my experience it sits at consistently 1mph below the set speed (as displayed on the dash) unless you are headed downhill. The vanilla cruise control probably has similar behavior, and the analog speedo could explain the confusion that OP is experiencing.
 

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I can chime in here as I am an expert on modern gps,... And also understand some basic math which I will explain.

Firstly, as mentioned above, your car has to meet a tolerance of 1.5 mph accuracy, so you are well within that, and therefore you have no claim.

Let's put things into perspective:
Assuming you are running 205/55r16 tires, you have a (new) tire diameter of 24.9 inches, which gives a circumference of 6.519 feet.
Now say your tread depth is 12/32" when new, so which would result in a diameter of 24.15 inches when bald, which would result in a circumference of 6.456 feet. So we are already at 1% error based on tread life alone.

But Wait There's More!
Despite government attempts to make tire manufacturers to standardize their sizes (and changing everything to the modern metric sizes), there are still HUGE variability between manufacturers, and sometimes between models from one manufacturers. A BFG 205/55r16 may be larger or smaller than a hankook 205/55r16.

Now about GPS
If I am going to make a blanket statement, I am going to say that GPS is the MOST accurate measurement of speed easilly attainable to the general consumer.

Now some caveats
Cel phones may or may not be equipped with GPS. Those not equipped with GPS will use radio telemetry (pings) between the nearest towers. This is horribly inaccurate, depending on your proximity to the towers. Turn your phone onto airplane mode to see if the GPS works, if it does, you have a true GPS antenna.
Now, as I mentioned above, the internal software may not be designed for accurate measurement of speed (specifically low end handheld GPS). If this is the case, the speed may not be corrected for your elevation, but gives your speed if you were at sea level (actually at the geiod, which is a mathematical approximation of sea level). As the earth is round, the further away you are from the center, the further you have to travel to make a complete revolution.
There are some other issues such as climbing or decending, not taken into consideration in GPS, but unless you are reading to 5 or 6 decimal places, these do not matter.

In short, your speedometer is an approximation of your speed, not a precision scientific instrument. As long as it reads less than the black and white signs, you have no problem.
 

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AAGG ;-)
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Mine 2016 3i is similar to the OP's. I set the cruise and measure, and the speed is either dead on or 1 mph higher than the setpoint at highway speeds.

Additional points of reference: my previous car (a 2007 Prius) and my wife's 2007 RAV4 both read about 1.5 mph (~2%) higher on the speedometer than actual. My 2002 Dogde Dakota reads dead on. My wife's 2007 BMW Z4 reads about 4% higher on the speedometer than actual.

Point is: even without issues related to tire diameter, each car can be a little bit different.
 
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