and then something strange happened

am I the only one to notice that after about 3000 miles, my fuel economy (as in total distance to one tank) jumped by almost 50 miles,
I was maticulous during the first 1000 miles, never running the engine above 3000 rpm's, and never breaking 70mph. Never going more than half throttle.
Has anyone else noticed this.
One the engine got to about 1500 miles, I took it out to the interstate, and really opened it up, pushed to an indicated 125. (still pulling really well at that point too)
 
Then almost like magic, as soon as it broke 3000 miles, the "miles to empty" display started reading 340 miles instead of 290.  

I'm always sceptical of computer readouts on fuel consumption, that sounds like a computer glitch (too dramatic a change). I'd go by a brim test to get real mpg.