City roads, yes, highways, maybe up to 20% above the limit, depending on the flow of traffic.
General rule I don't like speeding in the city since there are more blind corners, kids and traffic cams, so it's not worth it. In addition, with all stop signs and lights, constantly revving and braking hurts your mileage and brakes; plus it's harder to spot cops in the city as they blend in better.
On highways on the other hand, the speed "limit" is more of a "minimum speed", in fact if you're going under the limit on a highway you can actually be ticketed, since you're an obstacle for everyone around you.
I had that thing that plugs into the car from Progressive Insurance. Anyone else get this? The shit is very cool. It tracks your speed and breaking patterns whenever the car is started. You can pull up a report at any time as a graph. Discounts apply for safe driving patterns like speeding and hard breaking.
I have something similar; but it only measures 3 things for the discount:
-# of hard acceleration/braking incidents: This is the biggie.
-time of day you drive: Amount of time you drive late at night (12 - 6am-ish)
-total mileage: The less you drive, the greater the discount.
Interesting that whether you speed or not has no affect on your discount.
The discount is a max of 25-30%, which is pretty significant. I think most people average around 10%-20% savings.
I think the drawbacks are that its basically a black box; it keeps tabs on where you drive. You can even log on and put up a map where it tells you where you've been braking/accelerating. So if you get into an accident the info on it *could* be used against you (or exonerate you).