My sat nav uses gps and shows the actual speed, so if I’m doing 70 mph on the sat nav it’s reading 67 or 8 on the speedo

I've wondered about this often.

Here's what I figure.

Firstly, most cars have their speedometers set to be 2-3 mph faster than you are actually going. So if your speedo says you are doing 73mph, you are really only doing 70mph.

Secondly, when police calibrate their speed detector equipment at the start of each day they SURELY are making sure that it records ACTUAL SPEED.

So if it shows a car travelling at 70mph, then that car REALLY IS doing 70mph actual speed no matter what the car speedo says.

Thirdly, the police generally allow cars to go at the speed limit plus 10% plus 2mph.

So in a 30mph zone, you can be doing 30+3+2 = 35mph.

On a motorway you can do 70 + 7 + 2 = 79mph.


Putting all this together means that when looking at your speedometer you can go much faster than the speed limit.

For example, on a motorway, if the police won't stop you at ACTUAL speeds of up to 79mph then you can drive with your speedo showing 82mph (assuming it shows 3mph over actual).

This explains why, when you are on the motorway in those long stretches of 50mph limit roadworks, there are people whizzing past you all the time. You may think they are going to get speeding tickets but they won't.

If you are sitting there at what appears to be 50mph on your speedo you are actually only doing 47-48mph.

In a 50mph limit (and if police allow 10% + 2mph) then you can do 57mph before they will give you a ticket. That means on your speedo you can do what appears to be 59-60mph.


Things have changed somewhat though in the last year or so because speeding has been outsourced/subcontracted to private firms who have now created a multi-million pound business with speed awareness courses.

These private firms ARE NOT the police and WILL stop you at speeds just over the speed limit. Therein lies the problem now. I believe they would ticket you at speeds of 33mph and up in a 30mph zone.

They don't stop you, you just get an unexpected NIP in the post.

What sort of proof do they have to offer?

Yeah sorry JBR you misunderstood this one.

The private firms roam the streets in detector camera vans and park up in convenient places where they simply use laser equipment and camera to record your speed and take your photo.

If they deem you to have been speeding they don't stop you, they just report your offence and the first you know about it is the NIP (Notice of Intended Prosecution) that lands on your doormat. It gives you 14 days I think to declare the driver and decide whether to go on a course or take the fine.

They are scumbags, plain and simple.

They don't care if you're doing 33, 34, 35 or more mph in a 30mph zone. To them you're simply another customer for a £90 speed awareness course. The courses can take about 25 people and they are run on a daily basis.
So they catch as many people as they need to keep those daily places filled up.

Nice work if you can get it . . . !

"But I wasn't driving. I was at home that day. Yes, my car's outside right now. I think it must have been some joy riders who 'borrowed' my car. They've even brought it back in good condition. No, I didn't bother to call the police as I didn't notice it was missing, and in any event there aren't any police any more."

Unfortunately sir we have photos of the car as it came towards us travelling at 35mph and it clearly shows you are the driver. Therefore you are being issued with a speeding NIP and now, I am adding Perverting The Course of Justice to this. Have a nice day.

You are free to contest your case in court sir where the jury will be given copies of our photos. We will also supoena details of your mobile phone movements which I am sure will confirm that the device was with you in the car. You could end up spending a considerable amount of money on lawyers and court fees for what is otherwise just a simple £100 speeding ticket.