Most current speed limits were set in the 70's and it is time to bring them up to date.
We need to take into account modern cars with modern brakes and modern tyres and the fact that Pounder has reliably informed us the main reason for using a car is to travel in "Air conditioned comfort".
Since "Air conditioned comfort" is not speed related and the faster you drive the more fuel you use and the more danger you cause I propose:-

Inner city 8mph
Suburbia, towns and villages 12mph
NSL single carriageway 20mph
NSL dual carriageway 30mph
Motorway 40mph