Why not, LeSabres and Park Avenues are known for getting that kind of mileage on the freeway. I don't know much about the Riviera, but I can't see why it couldn't get the same since it has the same engine and basically weighs about the same.
I couldn't believe it, myself. I thought my gas gauge was broken when I first got the car. My Riviera regularly gets 31-33 miles per gallon on the highway. This car, when it was designed, spent many hours in the wind tunnel - very aerodynamically clean. I use my Rivi as our vacation car, and it goes to Vegas, quite a bit; matter of fact, 4000 miles round trip. This is the most comfortable car I have ever driven. Only draw back, it's easy to "speed" in this car because it's so quiet and comfortable.
Engineer in Wisconsin
Full member of the S.A.E. (Society of Automotive Engineers)
30-33mpg is very normal from a 3800 series II s/c'd engine. I had a 4-door GTP that easily got this mileage when I commuted in highway traffic without stop and go. City driving, where acceleration and the boost is used, is where you can expect 16-22mpg realistically depending on your habits.
UPDATE: I just fixed the wind noise! All I had to do was adjust the door latch on the frame and push it inboard about a quarter-inch. The door now shuts as solid as ever, and the window now pushes up tightly against the frame. I now have 161K on it and it still rides like a down pillow.
Unrelated to performance: I just put a new paint job on it, and went with a 3-stage, color changing paint going from deep electric blue to a nice emerald green. Definitely gets a lot of looks on the road now! In this case, it is polite to stare.