accuracy of magnetic field sensors in electrical vehicles
I will attempt to answer this question in the context of aerial electric vehicles, that is, multicopters in particular. Quadcopters maintain their position and orientation in space as a result of a tightly controlled feedback loop, which integrates measurements from accelerometers and gyroscopes (a minimum requirement), and sometimes even barometers, cameras, ultrasonic and LIDAR sensors. 

In the most basic case, the Inertial Measurement Unit (IMU) is able to generate reasonably accurate measurements, which is ensured by keeping it in the middle of the vehicle, away from the electromagnetic interference of the motors, and mounted on special gyroscopic foam, to prevent vibrations of the frame from contributing to noise. After an initial calibration step on a level surface, the quadcopter is usually able to maintain a reasonably good estimate of its angular orientation and absolute acceleration. The sensors may also be shielded, or interference can be reduced by a variety of techniques, including, but not limited to the following:


  On integrated circuits, important means of reducing EMI are: the use of bypass or decoupling capacitors on each active device (connected across the power supply, as close to the device as possible), rise time control of high-speed signals using series resistors, and IC power supply pin filtering. Shielding is usually a last resort after other techniques have failed, because of the added expense of shielding components such as conductive gaskets.


However, the above concepts may easily apply to a quadcopter, whose supply voltage is of the order of about 10 Volts, with currents of up to 200 Amps. However, full size electric vehicles will have voltages of several hundred volts and currents on a similar scale.

An article here discusses how this is still an area under research. This article is as of 2011, and the scene may be totally different today.