I typically drive 10 mph over the posted speed limit, both on freeways and on roads. IMHO, the posted speed limit is for either A) the driver with dementia who shouldn't be driving anyway, or B) some government that needs the speeding fines to balance their budget.
Go Los Angeles and there are some freeway offramps marked 25 MPH and, goddamit, they farking mean it oh holy shit will I make it. But as time goes on those honest speed limits get replaced with better intersections, but the speed limit stays the same.
Freeway speed limits should be 80. Non freeway speeds should be a good 10 MPH over what they are already.
Lucky you're not in Australia.. I have been booked (via hidden camera) for doing 64km/h in a 60km/h zone (39.8mph in a 37.2 zone).
Police generally will pull you over if you're doing 10km/h over the limit (6.2 mph) as the fine doubles at that point.
15km/h over (9.3mph) triples the fine.
And I'm not just talking about police on traffic duty - any police car will pull you over if you're speeding.
If you get caught doing 25km/h over (15.5mph) that's an immediate loss of license.
Our highway / freeway limits (apart from some isolated stretches on interstate highways) are all 100km/h (62mph).