I have done extensive testing under controlled conditions with the Prius and the difference between 35 PSI and 60 PSI was only 2.5%. In your case that would be 1 MPG. I have noticed a 3-5 MPG increase due to switching to smaller, lighter wheels with smaller, lighter LRR tires,
Compaq, I am dumbfounded by your results. A 33% increase in FE, IMHO, cannot be explained by a change in tire pressure alone, unless your tires were at 10PSI to begin with.
I can't explain it either but the more my tire pressure is the more mpg. I'm at 50psi now and my gas gauge hit 3/4 mark at 80 miles. With 55psi it hits at 90 miles. With 30 psi it hits at about 70-75 miles.
I think rolldown testing is how to measure this. On a flat stretch with no cars you take your car out of gear and let it coast down. If you know the weight of the car and how fast it slows down you'll know how much energy is lost, with no engine stuff getting in the way. This would be great for tire PSI, because it's so fast. The major problem is recording your speed vs time.