While something like a Scangauge can absolutely go a long way towards teaching you "how much acceleration is still fairly economical" and just exactly what kind of MPG to I get cruising at particular speeds, and things like that, you can still get a lot of good info out of the factory fuel meter on the Mirage.
First, you can get a lot of that sense of "what kind of MPG to I get cruising this speed" by simply achieving a speed, holding it constant, and resetting the meter. Keep driving for a few minutes, and the number you get will be very similar to the "instantaneous" reading that you might get from a Scangauge at that speed.
What I do when I'm hypermiling (which to some extent is almost always when I get into the Mirage) is watch that MPG display. It resets when you start the car (if it's been resting for 4+ hours), so you can start from that. Or... if you want to "cheat", you can reset it after the engine is warmed up... in the Mirage, that really doesn't make a HUGE difference like it does in some other cars.
So, I set out on a drive, and after the first couple miles, the meter should have settled into something fairly realistic. From there, I start playing the game. I want to move that number up as much as possible, and never DOWN if I can avoid it. You'll find (at least on a fairly short drive) that you can easily gain a couple tenths of an MPG every time you do a coast-down of at least 10 mph. I find that those coasts go on longer and end up being more efficient by doing them in neutral, at least in the Mirage. So, I do that "pulse and glide" thing. I'll gently accelerate up to maybe 5 over the speed limit (trying to NOT lose more than a tenth in the process), and then if traffic allows, I pop it into neutral and coast down to as slow as I feel comfortable depending on traffic and how big of a hurry I'm in. I might coast down from 50 to 30, or 45 to 25. And then accelerate gently IN TOP GEAR (unless you're below about 30-35, then you might use 4th... but, you'll be surprised what the Mirage can pull out of as long as the revs are above 1200) back up to speed.
You can also gain a couple tenths every time you stop by spotting the stop way in advance and coasting IN GEAR, and downshifting (I don't usually bother downshifting past 3rd gear) as you slow down to keep the revs above 1500. Doing so keeps you in DFCO (deceleration fuel cut-off), so you're using zero fuel for the entire coast. Then, after your stop, try to retain as much of what you gained as you can. If there's traffic and you've got to get back up to 50 mph, you might lose a couple tenths. You might also lose a couple tenths waiting at a traffic light. At the very least, most traffic light or stop sign stops should be a wash... you'll gain on deceleration, and lose on acceleration. But, if you just powered up to the stop and braked hard and then briskly accelerated away... you'd lost a lot more.
You can, without investing in any additional equipment, just use that factory MPG display to see real gains in economy. You just have to learn to play "the game". Every drive becomes an opportunity to set the high score!