I'm always surprised to see how many miles some of you are getting. My best tank came on an all-freeway drive between L.A. and San Diego...and I STILL got only 24mpg. In fact, when a tank consists largely of freeway driving, I'm averaging just above 20mpg.
I should note a few things here that may (or may not) account for any differences. My '91 is virtually all stock using OEM Yokohama tires, and of course it's a coupe so it's slightly lighter. I've gotten 28 mpg on a long trip, where I fill the car up, drive 400 miles or more at a fairly constant 80 mph, and then pull over to fill up again. So there are NO other stops on that tankful, and it's pretty accurate. On a 100-mile trip, you (a) might not be filling up immediately before getting on the freeway and immediately upon exiting, and/or (b) the mileage might not be quite as accurate because you're only putting in 4 gallons instead of 14-16. Just a thought, just trying to help figure it all out.