If you glance to your left while driving you'll see a dial with some numbers on it. When the needle points to the speed limit for the road you are on you'll not set off the camera.
Simple!

Exactly the situation I found myself in. I was in a Minor with a speedo calibrated for a 4.55 but running a 3.9 diff and I hadn't worked out the true (get me out of s**t) speed. I was being passed by another car at the time who definitely was speeding and the camera went off. Aparently they have the technology to read the speed of cars in all lanes simultaneously so I had to sit tight and assume the worst.Once you take into account the inaccuracy of a minor speedo