Ok, now all the scientific data.

You can't just simply assign a given number and say that your speedometer is always say 5-off, because the faster you go, the further away you get from true speed, making it a exponential equation instead of linear.  For instance, a quarter, because larger than a dime, would, at the same speed, cover more ground than the dime, but not have to turn as many revolutions.  And that quarter, would not only win the race, but the longer the distance traveled, the further apart they would get.  The same would apply to someone tall walking along with someone with shorter, stubby, legs- with cankles.

Anyway, the short version is that the faster you go with larger tires than stock, the further off and under your speedo will be.  With Dana's old speedo, each speed was progressively off until it read a good 6 mph over true speed.  Once the new gear was in place, her true speed was actually a hair under her MPH reading, and the difference stayed at a smaller variance, putting her safely at about 2 under at most common faster speeds.  This may mean that the 32 tooth gear would have been a better option, but the important thing is that now she would not be constantly wondering how much faster she was actually going than what her speedometer read.

Here is the data that was collected: proving the fix (but only reducing her ticket with the judge).

GPS unit used: Garmin Ique 3600

12 Satellite Receiver / Continuous Track

WAAS Enabled: Accuracy: < 3 Meters

Velocity: 0.05 meter/sec steady state

Actual accuracy confirmed at 8.76 - 7.74ft

So now that her speedometer is reading closer to her true speed, she'll only be speeding through Maricopa at a leisurely clip of 78.5 instead of a full throttled 86 MPH.