numbers are difficult subcompactor don't get discouraged, there's no consistency in just numbers. There are a huge number of factors that effect your MPG on the highway. Even without mods the factors you need to consider are wind, the number of cars around you, your tires and your ability to maintain throttle dead on. When i drive home from school i always get a "perfect scenario". the flow of traffic is moving at 70mph, there are a wall of cars at all sides of me to deflect air, the ambient wind is pushing me from behind, not fighting me in front, and there is only one slight incline. My numbers are obviously much higher moving in that situation than going against the wind, with no air deflection, and lots of hills. In that "perfect scenario" commute, at 70mph, i think ~3250 rpm, i should be getting around 50-55mpg constant depending on any number of factors. something as little as moving in the opposite direction on that same freeway would leave me at ~44 if i tried to maintain 70mph.
As far as mods go, i will say the mazda2 responded incredibly well to lowering, and i would say it's due to the horribly un-aerodynamic underbelly, mostly under the engine. Lowering a car encourages more air to move over it instead of under it, where it will cause loads of drag. From what i can tell the mazda2 is a great platform for fuel economy, the coefficient of drag is actually quite low. I have a prototype underbelly tray i was working on in the past in the garage, i need to get that back on and continue to do some more testing. Also pump up those tires, if you can handle slightly harsher road feel, go 50psi cold all around, but anything over the oem recommendation will help. I will say though, high pressure is a disadvantage if you ever need gravel or dirt traction, especially on a hill. Do not carry extra weight, don't use your truck as a storage compartment for your bowling and golf equipment when you only need them 1 day a week.
I wouldn't recommend trying to go lower than the speed limit on a 70mph freeway, especially if that means you'll be following trucks. A truck destroyed my oem windshield with 1 rock, and my second windshield has already taken 3 hits, something about our windshield attracts rocks. a $300 windshield expense won't make up for any short term savings in fuel economy. Continue what you do, but try to be consistent, if you have no way of monitoring throttle percentage i would recommend an ultraguage, they are very cheap and theoretically pay for themselves in the long run. On my ultraguage i track: Instant MPG, RPM, engine temperature(in actual degrees, which our car doesn't do), short trip MPG(average mpg since car was turned on last), throttle percentage, long trip mpg(average mpg of the car since i bought it) and loop status(closed/open).
If you do get an ultraguage, the sweet spot is 35.7% throttle, i really don't know what that number correlates to, as idling is 31%, and matted is ~62% iirc(i don't floor it too often!), but i try to never go over 40% unless its required, but for mainting speed on the highway, 35.7 is the way to go, 35.3 is slightly more efficient but it loses speed eventually, 35.7 maintains speed much easier and is still in the efficiency zone. This isn't always possible, when I'm going north and fighting the wind it usually requires at least 38% throttle to maintain 65mph.
