Surely it's just a matter of checking the little clock behind the steering wheel from time to time... if the needle's gone too far clockwise, you reduce pressure on the right foot... or am I missing something?
My car *wants* to go about 35mph in 30 zones, so I just do 27-28 most of the time as it's happy doing that. Hell, if you're that concerned, just put your handbrake on a couple of clicks, it acts like a sort of cruise control
It gets frustrating because lots of speed limits are highly inappropriate - often set because of a nervous council employee or a busy body who moved to a busy country lane and then decided they didn't like the traffic. I hate that. But, I obey the limits because I know if I get caught there is little excuse in "Sorry officer, I thought that was just a JOKE speed limit sign!". In 20 limits I invariably get overtaken by someone who hates the limit more than me.
There's three types of people:
1) those who believe any speeding, anywhere, any time is terrible and warrants being rogered with a red hot poker
2) those who believe moderate speeding is safe on the open road but carries a risk of fines
3) those who believe any speed they drive is fine and everyone else should do that speed too (but not down THEIR street whilst THEIR children are playing, are you mad?!)