I noticed a difference right away when I went from 20's to 18's.

I have my 18s on for one more road trip before I go back to my 20s.

my last trip I had 1500 miles, and I averaged 74.5 MPH (there are some 80mph speed limits in utah), with skis on the rack that added pretty significantly to wind resistance, and I got 24.5 MPG for the trip.

I have another trip this weekend where it will be just me, no skis, and a bit slower, with no 75/80 speed limits, so I'm expecting 26/27+ on this trip.

as a rule of thumb I was getting as high as 40 MPGs flat going 60 MPH, and would lose just about 1 MPG per 1 MPH increase after that in my 18s.

I'm looking forward to this next trip to see if I can approach 30.

combined city/hwy has been in the neighborhood of 21 MPG.

short answer to your question... I noticed a difference right away when I went from 20's to 18's.

20's go back on in 2 weeks, so I'll see if it was all in my head after that.