Then they have already subtracted their margin of error, which is definitely above 2km/h. More likely they saw e.g 66km/h in a speed gun with 4km/h error, so t

Then they have already subtracted their margin of error, which is definitely above 2km/h. More likely they saw e.g 66km/h in a speed gun with 4km/h error, so they know you did at least 62, and can book you stating 62. But most likely you were doing 66 or even 70 then.