White van man

[paranoid marvin]

My driving instructor said that you are allowed to add 10% to the speed limit if overtaking (and someone else said you can add another 2mph to that as well) so you can technically overtake someone LEGALLY at 79mph. Is there any truth to this?

No.

Speed cameras trigger at 10% above the limit to allow for variances in speedometers.

so when you think you're doing 70mph you may actually be doing 77mph already

[Dudley]

My driving instructor said that you are allowed to add 10% to the speed limit if overtaking (and someone else said you can add another 2mph to that as well) so you can technically overtake someone LEGALLY at 79mph. Is there any truth to this?

This is 100% bullshit.

The police can legally stop you for 71.

so when you think you're doing 70mph you may actually be doing 77mph already

If so your car is illegal, it's allowed to be 10% out in the other direction though. However yes, they account for the fact that analogue speedos simply aren't accurate.

My car indicates 3mph more than reality, yours is almost certainly close to that.

[Madge]

My car indicates 3mph more than reality

How do you know? Just curious.

[Dudley]

My car indicates 3mph more than reality

How do you know? Just curious.

Measured against GPS over a long motorway drive at night where we could try multiple speeds.

Not perfectly accurate I'll grant you but the consistency of result leads me to believe it.

[MC1]

My car indicates 3mph more than reality

How do you know? Just curious.

Measured against GPS over a long motorway drive at night where we could try multiple speeds.

Not perfectly accurate I'll grant you but the consistency of result leads me to believe it.

You could always find a local speed camera, and try a few runs past it at varying speeds.