Generally the rule is 10%+2. Above that and they will do you.

Bigger wheels will indeed make your speedo read slower than you are travelling, but seeing as most speedos read fast to begin with, it isnt usually a problem.

Checking against a GPS based product is best.


I would seriously look into whether the speed limit has changed recently. I know that there are quite a few roads lcoal to me, that have been dropped from 40-30, and in some cases national-30mph.

There werent any large signs to notify the changes, just tiny speed limit sign posts.

A few years ago I seen a speed camera van parked roadside on a national speed limit road. This was a dark wet night. Weirdest thing though, as in reality, even if you were doing 60mph on that road, you would be flying. Rarely would anyone be doing more than 50, as the road really doesnt allow it.
I stopped and questioned the officers in the van as to what speed the camera was set to record at ( police operate them here ).
he wouldnt answer. I then asked was he aware of the speed limit on this road.
He said of course they were.

Annoyed at the lack of answers, I went back to my parked car, some 60-70 yards behind them.
Within 5 minutes they were gone, and they had only been there about 10 minutes.

I'm pretty sure they were snapping away at drivers driving below the legal limit, because they werent aware of the speed limit on that road.