25 mpg sounds a little low.

Normally my highway mileage is between 30-31mpg. I saw 32 on one trip which was a good four hours of low speed cruise (45-50 mph back roads). After a tank of heavy foot in-city driving I still see 27mpg.
I did determind on my last long drive (4,700 miles) that the speedometer also functions as a fuel flow indicator. With the cruise set at, well let's just say high speed cruise, my highway mileage was around 27mpg.
I run redline, K&N air filter (no before and after numbers on this car with the K&N), and 37 front 42 rear tire psi.

87 SDL 282,000 miles