Did my first trip on the highway this week. I did about 563 miles roundtrip going from Grand Rapids MI to Milwaukee WI. This was a great way to measure realistic mpg on the HWY because you drive through 4 states with speed limits ranging between 55 and 70 mph. Plus there was a little bit of construction and Chicago bumper-to-bumper traffic thrown into the mix. Of the 563 miles about 25 of it was off the expressway traveling between the hotels, restaurants and customer locations. Here are my calculations, mods and information. Also just to clarify, while I believe the car's info commuter is fairly accurate, nothing is quite as accurate as starting with a full tank and then recording miles driven/gallons used at each fill up. This is how I measured it.
* 3.31 gears
* 6 spd manual
* Only modification at the time was my Bama tune (93 octane Performance Tune)
* 563.7 miles driven, 19.64 gallons of 93 octane used = 28.7 mpg (Car calculated it to 27.4)
Overall I'm very happy with those numbers.