Performed an experiment.
Filled up at the station around the corner from my house.
Drove down to work and back, Saturday morning, no traffic.
US-23 and M-59 -> Baker Rd and I-94 -> US-23 and M-59
Car sat at work for 5 hours.
After I start the car, I let it warm up until the RPM's drop down from the High Idle.
Shell 87 Octane
PSI: LF 42 LR 39 RF 41 RR 39
Total Trip: 67.2 miles
27.4 MPG Avg
55 MPH Avg
Sunny, ~75 deg, No Traffic, Highway speed was 70 MPH
Used 2.492 gallons of fuel = 26.96 MPG
I'll call that a win at 70mph!
2009 G6 GXP 3.6L Auto
1997 Jeep Cherokee 4x4 Auto 4.0L 241k Miles
Both on the road