How's this possible (from Top Gear's F10 M5 clip)

[E90potentialbuyer]

I was catching up on DVR for the Bahrain F1 Grand Prix (awesome race btw) and then clicked into my Top Gear recording. I paused at this. If you remember, this is Hammond driving the car in a drag race vs. E60 M5. Notice the digital speedo at 38 mph and analog speedo at roughly 45 mph. How's that possible? I haven't noticed this in my car. Perhaps I'm missing something? Or is this just off?

[E90potentialbuyer]

And here's a screenshot of a few seconds later into the drag race. Looks like Hammond switched the driving programs! Here the analog and digital speedo's tie out.

[M5Beast_NYC]

maybe the digital is slow to catch up.

[E90potentialbuyer]

Originally Posted by Carbonm3ny

maybe the digital is slow to catch up.

It should be the other way around right where analog is slow to catch up to digital? I have never looked at my analog or digital speedo's when driving hard (usually just see the heads up). But I find this very strange that the speedo's would be that off from each other.

[M6-Coupe]

The analog odometer is connected to the gearbox and catching the speed at real time speed. The digital odometer has a specific update rate and is always behind. Meanwhile, at higher speeds since the acceleration is slow, the digital odometer can almost catch up the analog odometer. You can take a video for 10 seconds from dig to 100+ mph to see what I said...

[E90potentialbuyer]

Originally Posted by M6-Coupe

The analog odometer is connected to the gearbox and catching the speed at real time speed. The digital odometer has a specific update rate and is always behind. Meanwhile, at higher speeds since the acceleration is slow, the digital odometer can almost catch up the analog odometer. You can take a video for 10 seconds from dig to 100+ mph to see what I said...

I see. Interesting. That could make sense - the digital speedo is not fluid so instead of showing every number from 0-60, it shows a few numbers. I guess the car was under hard acceleration and the digital speedo hadn't refreshed yet in top gear's case.

But I doubt that they a re connected to different sensors.

[allmotor_2000]

I doubt each comes from a different sensor - possibly digital moves in certain increments during quick accel (low speeds) and the camera happened to catch it before??

[E90potentialbuyer]

Originally Posted by allmotor_2000

I doubt each comes from a different sensor - possibly digital moves in certain increments during quick accel (low speeds) and the camera happened to catch it before??

Agreed. That wouldn't make any sense. I think it's exactly what you said (and what M6 was saying) - refreshes after set amount of time so under hard acceleration you can end up having a different readout in digital vs analog since analog is continuous.

[M6-Coupe]

Originally Posted by E90potentialbuyer

I see. Interesting. That could make sense - the digital speedo is not fluid so instead of showing every number from 0-60, it shows a few numbers. I guess the car was under hard acceleration and the digital speedo hadn't refreshed yet in top gear's case.

But I doubt that they a re connected to different sensors.

No they are coming from the same source but the digital numbers (any number on your dash) are coming from ECU after signal processing...

[Ex_Stig]

Top Gear is notorious for using filler shots on their show. I remember once when Clarkson tested the Veyron. Every time they showed the tachometer the car had the exact same number of miles on the odometer.

My guess is that those shots had absolutely nothing to do with the actual drag race.

[Boss330]

Originally Posted by M6-Coupe

No they are coming from the same source but the digital numbers (any number on your dash) are coming from ECU after signal processing...

Yes, they should both get their signal from the VSS, short for Vehicle Speed Sensor.

And both readouts are processed by the ECU.

On a side note, the digital readouts can be coded for accuracy and show exact speed. From the factory it's coded to show a certain percentage higher than real speed (for legal reasons and also in order to give you some tolerance in relation to public speed limits).

AFAIK it's not possible to code the tolerance in the analog speedo, only the digital display.

[M6-Coupe]

Originally Posted by Boss330

Yes, they should both get their signal from the VSS, short for Vehicle Speed Sensor.

And both readouts are processed by the ECU.

On a side note, the digital readouts can be coded for accuracy and show exact speed. From the factory it's coded to show a certain percentage higher than real speed (for legal reasons and also in order to give you some tolerance in relation to public speed limits).

AFAIK it's not possible to code the tolerance in the analog speedo, only the digital display.

Analog speedometer is a mechanical part with a set of small gears and bolts and nuts ... I doubt there is any control by ECU on this mechanical part; however all the data from all sensors are being transferred and processed by ECU...

[allmotor_2000]

Its a servo or electro-magnetic inside there - they stopped using cables and gears decades ago.

[Boss330]

Originally Posted by M6-Coupe

Analog speedometer is a mechanical part with a set of small gears and bolts and nuts ... I doubt there is any control by ECU on this mechanical part; however all the data from all sensors are being transferred and processed by ECU...

Originally Posted by allmotor_2000

Its a servo or electro-magnetic inside there - they stopped using cables and gears decades ago.

I doubt you will find many mechanical components like gears, bolts and nuts inside a BMW speedometer after the early 1980ies... The E12 (1974-1981) was the last 5-series to have a speedometer cable. And even those speedometers didn't have any gears They where eddy-current speedos where the speedometer cable/wire was driving a magnet inside the speedometer and as the rpm of that magnet changed the magnetic field caused the needle to move. The faster the magnet rotates, the further the needle moves up the dial.

The internals of a old school eddy current speedo which is used on cars with a speedometer cable:




Today the DME receives signals from the VSS and processes them into an output signal sendt to the speedometer. This signal is then converted to a movement on the needle, by similar means as the rpm counter or other dials in the dashboard.

For instance, the needle sweep test demonstrated in the video below wouldn't be possible in a electro-mechanical system driven by a wire from the transmission, like in the example above. Today it's a purely electronic system that receives it's speed signals from the VSS via the DME.