I drive my 318ic daily as well, 110-114 miles per day actually. Most of my driving is highway speed, with little traffic and only some traffic lights. I drive @ 60-65mph for 90% of the drive (posted limit is 55mph).
No mods have been done to my engine, everything is stock as it came from the factory (oops - with the only exception being that all of the under-intake hose stuff is removed).
At each fill up, I reset my trip odometer and make note of my total miles driven. Now, each time I have re-filled, my total driven miles based on the TO is 389 miles. Each re-fill has been 12.6 gallons of fuel. Based on my facts, I've been netting 31.12 mpg, which to me is good for a 17yr old vehicle. My total miles driven and re-fuels are identical every single time, only because my route to & from is the same on a daily & weekly basis.
Now, when I do go for the re-fuel, my gas gauge needle is just about on the first white line @ the "R". I've read that once the gas gauge light comes on warning of a nearly empty tank, there are at least 12 miles to go before the tank becomes really "empty". If I'm getting 31 mpg, I'm sure I could possibly stretch out my remaining fuel so that I would get about 395-400 out of a single tank of fuel, but that is risky, so I will never attempt to do that... LOL.
It seems most 91/92 318's get anywhere from 25mpg-32mpg based on online forum responses.