How do nonlinear speedometers work?

Some cars, now have nonlinear speedometers. How can they achieve to get different spacing between some speed ranges?

For better understanding, see the image

of an old Audi A4 speedometer.

The range from 70 to 100 is not linear, because there are equally spaced marks, as you can see in the table:

``````Mark        Increment from previous
70               5 (was 65, not shown on this table)
75               5
80               5
90              10
100              10
``````
• Not sure I understand what you mean? Can you explain what you mean by "spacing"? Feb 13, 2016 at 14:27
• I expect the answer will be signal processing. Rather than display the speed which corresponds to the voltage output of the speed sensor, run it through a piecewise function
– Zaid
Feb 13, 2016 at 14:27
• @ᴘᴀᴜʟsᴛᴇʀ2 Something like this
– Zaid
Feb 13, 2016 at 14:30

1 Answer

The speedo will be a standard linear voltmeter.

Vehicle speed sensors are usually attached to either the gearbox output or the differential and produce a pulsed output at a frequency proportional to the vehicle speed.

On a 'normal' speedo, the ECU will read the pulses from the speed sensor and produce a linear voltage proportional to the vehicle speed to send to the speedo.

With this non-linear speedo, the ECU simply creates a non-linear voltage range that fits with the graduations on the speedo. This is a very simple task to perform in software.