I would not call the time base setting a speed but a capture time. 20ms/div is a 200ms capture time. The scope speed would be how often the scope would sample over that capture time. There are several settings that affect the sample rate speed (samples per second) depending on which scope you are using.
The frequency or speed of a sensor signal would depend on the type of sensor and the engine RPM.
The important thing here is that the scope speed (sample rate frequency) be sufficient to handle the sensor signal speed (signal frequency) to reconstruct the signal with accuracy.
The PicoScope automotive scopes are unique in that they can accomplish very long capture times while maintaining a robust sample rate. This allows the technician to capture relatively high frequency sensor signals over long periods of time without loss of waveform detail. You can still zoom in and examine the waveform detail down to an electron microscope level.
No other automotive scope can come close.
(The Picotologist) http://www.autonerdz.com
THE PicoScope Automotive Authority
In North America