My freeway commute is 36 miles each way, about 33 of those miles on I-10 at an indicated 70 mph. I very consistently get 67 mpg. Only changes I've made are the better plug and an 18T front sprocket. However, mileage did not noticeably improve when I had the sprocket installed, so I don't think that made a difference.
I have a B5 also, which has 19" wheels front and back. I know the speedometer reads high (indicated 70 is actually 65), but I don't know if the odometer is off by the same amount or not. If so, then the actual mpg is less.
Notice, my "fuelly" tag below says 64.4 mpg, but that includes all tanks since new. I used to go faster all of the time! (I used to run indicated 75 mph+ to keep up with traffic. Now I just accept that I'm the slow guy on the freeway. The speed limit is 65 anyway, and I have enough company at the slower speed that I don't feel like I'm holding things up.)