Is it possible for a 2006 Toyota Prius used with 143,000 miles on it to average 59 miles per gallon on highway driving but now I was driving at my sweet spot which is 55 mpg
Is that MFD reported or manually calculated? The MFD's have been know to be a little "gracious" in their fuel economy estimations. But even if MFD indicated, I'd suggest you are actually 54 mpg which is still excellent. The right weather, road, driving and vehicle conditions can yield these results. Keep it up!
Yes definitely, especially if you use low rolling resistance tires inflated to maximum sidewall pressure and 5W-30 or thinner oil. Also, the slower you go, the better mpg you'll get on the highway as wind resistance increases exponentially with speed. Some users did testing on a 3rd gen: Updated MPG vs MPH chart | PriusChat The results for a 2nd gen Prius should be similar but ~6% lower (as the 2nd gen is rated ~6% lower via EPA). Looking at that chart, I think you should be able to get 55mpg easily at 60mpg.
