Speedometer recalibration?

[ME2SC]

I've noticed that my speedometer is reading 2mph over, when measured against GPS and limit sign radar displays. Reads 35 at 33, 55 at 53, etc.
Not a big deal, but a little niggly thing that reminds me of a pet peave regarding people setting clocks ahead to the incorrect time so they won't be late. Ironically, they typically are. I can't stand that.

Anywho, I wonder if anyone else has experienced a similar speedo discrepancy and if so, is there a way to correct it?

 

[EvanstonI3]

They are all set this way.

 

[mojo]

It's impossible to get complete accuracy, because roadspeed is still calculated on the basis of wheel rotations, so its accuracy will change as the tyres wear, and will also vary slightly by make/type. Most people would not want to have to recalibrate their speedo every time their tyres are replaced, so a degree of over-reading is considered inevitable.

Unfortunately GPS speed can't be considered accurate either, because most GPS devices don't account for gradient.

 

[Neil]

I've noticed my speedometer has always read about 2 mph fast since I've owned my vehicle purchased in 2014. Most of the time I can make the correction in my head

 

[EvanstonI3]

Every BMW I have owned or driven in the last 15 years has done this exact same thing.
Every Toyota I used to drive was also like this, although it was not quite as noticeable because they did not have Digital Speedometers, but when driving very long distances it became apparent while being bored and mucking about with tracking milage and GPS and whatnot.

 

[alohart]

You wouldn't want the speedometer to read lower than actual which would increase the risk of speeding tickets. As tires wear, the speedometer reading increases at the same speed because the tires are slightly smaller in circumference. So the speedometer error would be the least when the tires are new. If tires with greater new tread depth compared with the OEM tires were installed, the speedometer might actually read lower than actual. BMW seems to have calibrated the speedometer so the risk of it reading lower than actual is minimal.

 

[ME2SC]

For a tire to be worn enough to make a deviation of even a single MPH seems pretty extreme, as in: from brand new to legal remaining tread depth. As someone who has never been handed a single speeding ticket in over 40 years of driving, I'd be happy to have it read as accurately as possible. Unfortunately, that seems to be unavailable, so I s'pose I'll be content in about 20,000 miles or so ... B)

 

[ME2SC]

Unfortunately GPS speed can't be considered accurate either, because most GPS devices don't account for gradient.

I account for this and comment on readings I've had on flat stretches only.

 

[Dutchi3]

Regarding speeding tickets, there is always a margin/deduction of the speed measured by the police so you have some wiggle room there as well. Almost everyone knows ;-)

 

[Whereitis]

I have a Garmin that I rely on for speed and other traffic and road data, since the speedo in my i3 typically reads about 3mph low at highway speeds, and the i3's nav is sadly out of date. Let's hear it for free lifetime map updates!

 

[mojo]

Let's hear it for free lifetime map updates!

While I understand what you're saying, the truth is that "free" map updates are never free. You just pay more upfront, built into the cost of the device/car.

 

[wjsi327]

I've noticed that my speedometer is reading 2mph over, when measured against GPS and limit sign radar displays. Reads 35 at 33, 55 at 53, etc.
Not a big deal, but a little niggly thing that reminds me of a pet peave regarding people setting clocks ahead to the incorrect time so they won't be late. Ironically, they typically are. I can't stand that.

Anywho, I wonder if anyone else has experienced a similar speedo discrepancy and if so, is there a way to correct it?

I've noticed that my speedometer is reading 2mph over, when measured against GPS and limit sign radar displays. Reads 35 at 33, 55 at 53, etc.
Not a big deal, but a little niggly thing that reminds me of a pet peave regarding people setting clocks ahead to the incorrect time so they won't be late. Ironically, they typically are. I can't stand that.

Anywho, I wonder if anyone else has experienced a similar speedo discrepancy and if so, is there a way to correct it?

My 2021 i3 BEV, 2018 i3 Rex, and my 19

I've noticed that my speedometer is reading 2mph over, when measured against GPS and limit sign radar displays. Reads 35 at 33, 55 at 53, etc.
Not a big deal, but a little niggly thing that reminds me of a pet peave regarding people setting clocks ahead to the incorrect time so they won't be late. Ironically, they typically are. I can't stand that.

Anywho, I wonder if anyone else has experienced a similar speedo discrepancy and if so, is there a way to correct it?

My 1997 Z3, 2018 i3 Rex, and my 2021 i3 Bev did and do exhibit the same thing. Every BMW forum I’ve read makes the same claim that it’s on purpose. Supposedly, because a German law prohibits manufacturers from under reporting the actual car speed, BMW deliberately sets their speedometers to read a couple mph, or kph, high. But does that mean that the odometer is off too? The dealers will tell you no. And what about other German cars? Do they all do this?

 

[Whereitis]

The Garmin I have in the i3 was bought new (several years ago), for advanced features such as its built-in dashcam. But in my other vehicles I have Garmins bought for $40 on Craigslist that all have lifetime maps and I will never buy one that doesn't. If you go by the sticker price they're all overpriced, but I would never buy a new i3 at sticker price either.

 

[mojo]

Supposedly, because a German law prohibits manufacturers from under reporting the actual car speed, BMW deliberately sets their speedometers to read a couple mph, or kph, high.

As I understand it, this is not unique to Germany. I believe the permitted range is -0% to +10% within the whole of Europe (and the UK). Of course, they can get much, much more accurate than that for one specific make/size of tyre, with a known level of wear and a given tyre pressure - but different people fit tyres of different sizes, so there is little point in them trying for extreme levels of accuracy.

On my other vehicles which over-read by up to 8% I have fitted a standalone GPS-based speed readout - though, as said, GPS can be inaccurate on hills and also during turns.

But accuracy to 2% is good enough for me on the i3 - I can figure out in my head roughly how much leeway I have before I actually hit the speed limit.

 

[JamesInLondon]

My Waze gives me an accurate GPS calibrated speed and I use that speed to set my ACC, the speedometer is purely an approximation. In the i3 it seems to be around 6% optimistic; in my Mercedes A124 (which had bigger wheels, but lower profile tyres than standard) it was around 8% optimistic and in the BMW X3 I rented in Italy recently it was about 5% optimistic

 

[jfran2]

My 2021 i3 BEV, 2018 i3 Rex, and my 19

My 1997 Z3, 2018 i3 Rex, and my 2021 i3 Bev did and do exhibit the same thing. Every BMW forum I’ve read makes the same claim that it’s on purpose. Supposedly, because a German law prohibits manufacturers from under reporting the actual car speed, BMW deliberately sets their speedometers to read a couple mph, or kph, high. But does that mean that the odometer is off too? The dealers will tell you no. And what about other German cars? Do they all do this?

FWIW, my wife's 2018 Mercedes GLC300 speedo reads right on the money, at least according to a Garmin GPS unit. My 2004 Saab reads 5 mph fast.

John Francis
Rolla, MO

 

[amalcolmm]

Supposedly, because a German law prohibits manufacturers from under reporting the actual car speed, BMW deliberately sets their speedometers to read a couple mph, or kph, high.

I noticed the discrepancy a few years ago and wondered where it came from. A certain inaccuracy could be explained by the tire/wheel not quite matching the programmed conversion constant in the firmware, but for it to be a constant offset of ~2mph really had me stumped. Why have something that is dead-nuts accurate to then have a constant error? This would explain it.

 

[Whereitis]

I'm a little disappointed in this. The speedo of my last car, a 2014 CX-5, was spot-on with the Garmin for 3 years. Now with the same Garmin in the i3, on the highway the speedo consistently reads 3mph high. A friend who works on his own cars & owns 4 Mercedes says there's a screwdriver adjustment for the calibration, but I haven't found it.

 

[mojo]

A friend who works on his own cars & owns 4 Mercedes says there's a screwdriver adjustment for the calibration,

That probably suggests that his Mercedes cars are a few years old! When speedos were mechanical devices, this was possible - easier on some cars than others.

Then they changed to pulse-counting and it got a bit harder. When I changed the auto box on my late 80's Jaguar to a manual Getrag box, I had buy an interface box to alter the pulse count and correct the resulting inaccuracy in the speedo (made by Dakota Digital - they still make them 20 years later!).

But now they are entirely digital. Typically the car's ECU will send speed data over the car's CANBUS (or similar) network, and the dash cluster will interpret that and display it. There's no room in that system for a screwdriver adjustment.

Having said that, some manufacturers do allow for speedo adjustment to account for different sizes of tyre. VW, for example, incorporate several different tyre size "profiles" in the dash of some of their models, and it's possible to alter the coding of the cluster to correct the speed readout. But they don't allow you to correct the reading in (for example) one MPH increments. I suspect that this is mainly due to processing limitations in the cluster itself - it's not designed to do that sort of corrective mathematics in real time (but I should add - that's just a guess on my part).

 

[eNate]

I've heard the story about German laws, too, and not only does 3 MPH sound excessive at highway speeds, but my German-built VW ID.4 speedo calibration is spot on (when compared to GPS – which is what other ID.4 owners also report).

So there's got to be more the story.