Let me try summarizing what's available in the public for literally half a century or so, in no particular order. (There may be some overlap with other answers.)
Optimal speed
As others already mentioned, this is just fantasy. Whatever the value is, and it exists, you won't be able to apply it in real-life driving situations. E.g. in the city you'll tend to be too fast, on the highway too slow, citing the speed you mentioned.
So what are alternatives?
Don't drive
It's less silly than it may sound. Commuting may be the biggest part of consuming gas, but there are many more routes we take. So, think about avoiding using your car more frequently e.g. by:
- better planning (one round trip instead of repeated starts from home)
- team up with colleagues to drive together and agree about cost sharing (which will pay off quickly)
Tire pressure
Check regularly, i.e. more frequently than once a year.
Flexing tires with every rotation consumes considerable energy. If pressure is too low, this amount increases, resulting in more gas consumed per mile. Overpressuring a little isn't harmful, saves gas and avoids wear-down pattern typical for flat-footed tires.
Your cars mass
We all have the tendency to drive around extra luggage, like blankets, the old battery etc. Inspect your cars for items which don't need to follow you each and every day ... and take them out. Keep all legally required items (and make them complete again, while you're at it).
Look ahead ahead
The way you drive is amongst the biggest contributors to gas consumption. Typically you have these three situtions:
- start driving (accellerating, $a > 0$)
- driving at constant speed ($v = const., a = 0$)
- braking/stopping ($a < 0$)
#1 and #3 are the gas killers, while #2 is a steady-state, where gas consumption is much smaller. BTW, time is your friend: you don't need to drive a racing car ($v = a \cdot t$), which drives your MPG up, up, up. So keep $|a|$ small whenever possible.
So the further you can watch geometrically (1-2 miles ahead, at least) and in time (anticipating traffic), the more often you can avoid or reduce the impacts of #1 and #2. Besides your eyes use traffic information. "Read" the other vehicles around you. Etc.
In traffic jam avoid stop-and-go. Increase your distance to the car in front of you, and try keeping a constant low enough speed instead, i.e. keep rolling, flowing, where others don't. With a truck in front of you, watch e.g. shadows ahead or traffic lights to gradually speed up in time, i.e. often a bit earlier than the other impatient "lemmings".
Transmission and gear
I assume you don't have an automatic: then there is little you can do, besides e.g. lifting your foot from the gas pedal to signal the system "there's a change". However, Cruise Control can literally pay off.
With manual transmission you can do many things, with situations #1, 2 and 3 in mind, e.g.:
- when accelerating, switch gears earlier than usual (roughly, consuption is proportional to $N \cdot v$, where N is your motors rpm)
- for constant speed use the clutch and reduce gas: your car will roll even with least RPM
- watch topography: rolling down the hill literally asks for opening the clutch; gradually increase speed in time when approching up-hill
- this implies using your highest gear as soon and as often as possible
- when braking, open the clutch and brake, while keeping gear: often using rolling- and air-resistance from an opened clutch is all you need; adjust gear once you know you need to accelerate again
Safety
BRAKE hard and fast (OPEN clutch AND NO off-gas, a two feet operation) when you need to: time lost never returns here, i.e. results in accident. (Yes, and watch the "lemming" in your back always, best in advance).
If you haven't done yet, attend a safety training, which will open your eyes beyond MPG.
Self-feedback
Track you mileage somehow. If you car offers a measurement for MPG, try using it. Else, refuel more frequently, at least for a while, and divide your required gas volume by your miles driven. From your protocol you also know your behaviour. Try learning from it, besides seasonal impacts (more gas needed in winter). E.g. detect your preference for driving with open windows, which eat up gas, and change it somehow.
Repair etc.
Keep your car intact. Listen to their statements regarding your motor, oil, cat and what have you. Try reading the repair list as a forensic statement, telling you about your car, its traffic and ... yourself.
Refueling
You probably know and use it already: gas price varies much during the day, from day to day etc. Use websites or apps to anticipate the best time for refueling now. You don't always win, but will save money overall.
Final
When we were driving with manual transmissions, my MPG was constantly better than my wifes: she just drove.
With our automatic, my MPG still was better, because I used the Cruise Control more frequently than she does.
With the dramatic increase in price from ruZZia's illegal invasion into Ukraine, she found shorter routes and avoided using the highway without sacrificing arrival time ... which reduced her MPG.
