With or without magnets, in the heat of battle (race) it will happen enough times that more than one person will get frustrated by it.
The main problem is that your placement RELIES on a specific angle for the cars you've tried. Longer or shorter cars will be at a different offset. It might not matter, of course, but the speed will matter. Slower or faster.
The best option, if possible, is to have it so that the racers, knowing their car is straight as it passes over the sensor, need only worry about the actual speed limit of the fuel sensors, which I believe is 50% throttle, and not how fast they might be traveling as they cross over that sensor.
That said, sometimes choices are made and the ramifications are simply dealt with.