Actual speed on the instrument vs. shown speed at the MID

[peake]

This is something that has puzzled me since I bought my 850i -92. The speed shown on the meter, say 100 km/h (62.5 mph) is shown as 92 km/h (57.5 mph) on the MID (Hidden menu 8). The signal is likely to origin from the sensor in the final gear, is it not? I don´t know for sure or are there more than 1 sensor giving signals to the systems? Also is there any correction factor that can be adjusted to set the 2 indications equal and closer to the actual speed (Gps speed which is also different from these 2)

//Peter

[TxGR8White]

Speedometers almost ALWAYS show just a little bit faster than the actual speed. This is to put any speeding liability on the driver, not the manufacturer. If the speed limit is 120kmh and the speedo shows 120kmh, but you are actually doing 125kmh, you can say that it is BMW's fault since YOU thought you were doing the speed limit.

[m6bigdog]

The speedometer error is a feature. I always assume somewhere around 5-7 MPH low at 60 MPH.
See attachment:

Tire diameter and revolution per mile is not a standard and will vary by tire manufacture, tire model and tread wear.
The MID speed reading is an unconditioned reading based on the programmed tire revolutions per mile.
However, the MID reading may be in error -/+ of actual as the actual tire revolutions may not be the same as the revolutions per mile programmed into the electronics.
Hence, the only way to determine the actual vehicle speed is to measure it with an independent speed system, GPS, Radar, Lidar or other speed reference device.

Drivers that religiously follow the speed-limit based on the speedometer are constantly driving below the speed limit!! And, they wonder why there is a long line of cars tail-gating!!

[Dr. T]

So, do they get their measurements from the same sensor are there 2 different ones? And which does the odometer follow? The speedometer reading or MID?

[ByMW]

There is one sensor at the output shaft of the differential, right side. All speed info displays are electronically derived from that input. I don't know if the odometer display is "dumbed down" like the speedo gauge, but I would guess not - in other words, I suspect it uses the best info available so the mileage is as accurate as possible. Just as the speedo gauge would err on the high side for liability concerns, the mileage value would not want to appear artificially low.

[m6bigdog]

So, do they get their measurements from the same sensor are there 2 different ones? And which does the odometer follow? The speedometer reading or MID?

Just the one magnetic read-switch sensor in the diff.
The speed signal is processed and distributed from the Body Control Module(EKM) to the various systems that use a Speed data, i.e., Instrument Cluster, DME, Throttle, Cruise Control, EDC, Radio, etc.
The Instrument Cluster Odometer and Speedometer Displays are transferred over the Instrument Cluster Data Connection (data-link) from the EKM.
The MID display also comes from the EKM but over I-Bus data connection.
That I can tell from the ETM Diagnostic Manual, the Features for the Instrument Cluster analog Speedometer Display are added in the Body Control Module to the speed data transferred to the instrument cluster .
The I-Bus speed data and odometer data transfers remains unconditioned as does the Speed A signals.

[ChrisFuture8]

My car has the complete opposite speed indication. The speedometer gauge would show about 8mph slower than what the MID shows. Not sure why but I think it may have something to do with the instrument cluster swap from (originally in kilometer) Canadian to US model. I'm always doing like 80mph when the speedometer shows 71'ish. I always have to remember that when I'm driving the car.

[TxGR8White]

My car has the complete opposite speed indication. The speedometer gauge would show about 8mph slower than what the MID shows. Not sure why but I think it may have something to do with the instrument cluster swap from (originally in kilometer) Canadian to US model. I'm always doing like 80mph when the speedometer shows 71'ish. I always have to remember that when I'm driving the car.

What size tires are you running?

The speed sensor picks up the axle speed and converts that based on the original tire sizes (circumference) to arrive at actual speed (+/-). If your tires are too "tall" i.e. the circumference is greater than stock, you will be travelling "faster" than what the speedo shows.

[ChrisFuture8]

The car is currently rolling on 17s. I thought the smaller tires would actually register higher speed.

Sent from my SM-G955U using Tapatalk

[TxGR8White]

The car is currently rolling on 17s. I thought the smaller tires would actually register higher speed.

Sent from my SM-G955U using Tapatalk

It is not just the wheel diameter. The aspect ratio is what will make the difference. For example, a 245/45/17 tire is "taller" than a 245/40/17 - the later will produce a lower "speed" with the same axle RPM. Same applies for 16, 18, 19, 20, and 21" wheels...

[ChrisFuture8]

Hmmm, I see. Thanks for the clarification. So for folks who have upgraded to larger size tires greater than the original 16s but possibly with lower profiles, are there any calibrations that could be done to ensure speed indicated on the speedometer is accurate? How about folks that went to 19s and even 20s?

[peake]

I did a short trip to day measuring the speed. I had the speedometer, the mid and also a gps speedometer. I did set the cruise control at 3 fixed speeds 50, 70 & 90 km/h.

Speedometer km/h	MID	GPS
50	43.2	46
70	62.2	67
90	82.2	87-88

BTW my tires are 255/55 on 16" rims.

[tangcla]

It is not just the wheel diameter. The aspect ratio is what will make the difference. For example, a 245/45/17 tire is "taller" than a 245/40/17 - the later will produce a lower "speed" with the same axle RPM. Same applies for 16, 18, 19, 20, and 21" wheels...

See here for a comparison on tyre profiles and their corresponding diameter (or 'tallness' as described by TxGR8White) and the effect it has on the speedo.

Simply comparing two 'standard' tyre sizes for the 17 inch wheel - 265/45/17 and 275/40/17 - results in a 2.8% variance in speedo reading as the diameter changes by 18mm.
Here's the comparison table with dumb imperial measurements.

Specification  Sidewall  Radius  Diameter  Circumference  Revs/Mile  Difference
275/40-17         4.3in  12.8in    25.7in         80.6in        786        0.0%
265/45-17         4.7in  13.2in    26.4in         82.9in        764        2.8%