I'm about to do a 2,400 mile round-trip so I'll get some numbers. Just picked the box up last week.
I upgraded to the 2020 GTR from a 2017 GT last February. Due to pandemic I only just broke 3000 miles. Up until now have been mostly stop and go city and have been getting 20MPG. My old GT averaged 23-24 in same conditions.
I hit short term 24MPG on fresh tank on my way to pick up the box and maintained a little under that on way back but was stuck in rush hour on I80 and GSP
Okay, so got back from my trip. Averaged about 24.7 MPG with average of 78MPH.
The worst section was leaving Chattanooga going towards Montgomery. The road was wide open with a lot of head/cross winds and got roughly 22.8 MPG, even though I think we were mostly going down hill out of the mountains.
The best was Kingsport, TN to NJ and averaged 27.2 MPG. Tailwind and mostly downhill helped a bit.
Going NJ to Wytheville, VA averaged 25.2 MPG.
MPG Computer was consistently under calculating based on refill amounts with variance of .6 to 1.2 MPG.