Sorry, friend, but it can. If you have a scan tool that tracks real time fuel delivery and speed you can easily see the 'sweet' spots for the Si (40-45mph in 5th gear, and about 62-65 in 6th gear). Both can return over 40MPG on a relatively 'flat' roadway. Once you hit over 70mph you MPG drops dramitically. At 80mph you are certainly sacrficing fuel economy for speed. Additionally, cruise control is 'required' for those MPG goals. It can make minute adjustments in fuel delivery to make very small adjustments that your foot cannot attain.
I have personally gotten over 38mpg on a trip to California from Vegas, which is inclusive of a few big hills. Given that trip, I feel quite confident that I could attain 40+ should I ever have a road trip across Oklahoma or TX. There are a few other forum members with scan tools that use them when on road trips in their Si who have seen very similar results.
You will never seen that kind of mileage if you have mixed driving (city/hwy), driving in hilly country, accelerate agressively, or drive at over 65mph'ish in 6th gear. This is not, however, what the Si is about, which is why you will normally see about 25mpg on the average, lower if you are scooting about in a spirited fashion. Getting 'hypermileage' in your Si is a 'hobby' or 'challenge' thing for a few strange folks like myself who like to see 'what you
can do'. Having seen the results of experimentation, I have changed a few habits that have increased my 'overall' mileage quite a bit, but, as I said, I typically wreck it with weekend bouts of spirited driving.
And, as a point of reference, you can achieve well over 300MPG coasting downhill at 60+MPH out of gear. (Again, scan tools are great...if calibrated correctly)
There are many nay-sayers and non-believers, but those of us who actually have the scan tools to see what fuel delivery is taking place know the truth. It can be done...quite easily at that. But you have to be patient and dedicated. (and be willing to be bored at 63-65mph on road trips)