I check my mpg @ the gas pump. I was @ 288.4 miles yesterday, & it took 6.02 gallons to fill up (second click of the handle). 288.4/6.02 = 47.9 mpg. Most of that previous tank was an approximate 5-hour round trip going 60-75 mph depending on the highway (mix of state, county, & interstate). It was cool enough in the morning hours to drive with my driver's window open, & two rear windows open a a couple inches. I seldom use A/C, unless it gets really hot or want to listen to the radio better while on the highway. I could have easily gone another 120 miles before filling up, but I needed to make a bathroom/grab something eat stop, too! With COVID-19, I try to simplify/reduce my number of stops when traveling.
As far as squares left on the gas gauge, that has nothing to do with mpg other than your car is lighter with less gas in the tank. You could use all 8 square without issue, but I wouldn't stretch much beyond that.
I think of each bar (8 squares) as being approximately 1 gallon of gas. If you use up all the 8 bars, there seems to be about 1.5 gallons of gas left. If you haven't tested this out, the last bar will start flashing at some point. Even at that point there is no reason for alarm, unless you are 65+ miles from a gas station.
There is nothing wrong with adding gas at two or three bars. That's probably a good practice, but it's not impacting your mpg in a negative way. Something else is going on here? I never see below 40 mpg in the summertime with my 2017 ES manual. I tend to be above 45+ mpg most of the summer (last three tanks were in the 46.8-47.9 mpg range), which is about typical. Winter averages drop around 42-43 mpg on most tanks. I never look at the mpg gauge on my car. I calculate at the pump.
I never look at the economy light on the dash or the tach. I shift when it time to shift, whether it's accelerating or climbing a hill.
l ordered a regular spring from summit racing and got this...going to order a small spring to see if l can somehow mount it under the pedal...just like in the photo, l can only adjust one way, which is not good if l need speed.....probably better to use my foot lol. Basically l want to mount a spring underneath so it makes it harder for me to push WOT, but we'll see.
I may not understand the issue here? I find nothing odd or different about the gas pedal on a Mirage. Yes, the clutch is lighter than most manuals, but the gas pedal seems pretty normal to me.
As a driving instructor, I work with students who have very little driving experience. Mastering the touch of the gas pedal is not a huge issue. I realize a manual complicates things, but it shouldn't take an extra spring to improve the driving of one.