My '94 TT BPU(RMM DP, BCC, Greddy SP, HKS intake) 6-speed would get 26-27mpg if I kept it under 80. I'd drop to mid 20's if I broke the 80-85mph mark. I always ran premium fuel so I don't know if regular unleaded lowers the mileage or not.
I bought the car in Atlanta, Ga and drove it home to Denver, Co. On that trip I drove nearly 1500 miles in 26 hours. I was commonly driving 100-130mph out on open Interstate and I averaged 24.8 mpg over almost 1500 miles. Of course I had to drive the speed limit through cities and what not but overall I was impressed considering the speeds I was running.
I have the tickets to prove it. Two Kansas state troopers wrote me tickets, 5 minutes apart @ 3am in the morning. The first for 103mph in a 70, the second 107mph in a 70, just a few miles down the road.
NEVER turn your traction control off when pulling away from a Kansas State trooper. They don't enjoy having gravel and shredded rubber tossed at them as you pull away under full boost.
Just my $.02