Google is your friend - there are a lot of resources on the net about how to best combat a ticket - all kinds of interesting information including information regarding illegal speed limits (in some states if the speed limit for that part of the road was not formally evaluated by the DoT then posting signage may actually have the effect of making the speed limit completely unregulated).

The sites may not directly address the usage of a scan guage, but it can probably be used as supporting evidence for inaccuracy of measurement, depending on the other conditions.

One thing to note - "settling" for a reduced fine is usually bull****. The cost of a ticket is rarely in the fine - it is in the long term effects on your insurance rates - which can easily cost you thousands over the course of a couple of years. In *most* cases you would be better off paying a higher fine up front in exchange for keeping the affair off your driving records.

Even if your speed was dangerously in excess you should do everything you can to fight the ticket - far too many speed limits are all about revenue and actually decrease safety because they are set too low. So no matter how "guilty" you might feel, consider all the poor slobs who got screwed over by illegitimate speed limit enforcement and consider it your duty to try to balance things out. Speed limits shouldn't be a game, but its the politicians who have made it that way so you should feel no compunctions about playing hardball.