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, **** 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 **** 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).