Fuel pressure running or key off will be in a range, not a specific number, for a few reasons, one of course is the pressure gauge, lol, these are not calibrated daily, so grain of salt
Yes, 55 to 75 is expected, and as long as it steady at idle any number in between those is fine
Raise RPMs to 2,500 or so and hold there, should also see a steady pressure, slow drop as you continue to hold higher RPMs means clogged fuel filter or failing fuel pump
You should see a 10 to 20psi drop in fuel pressure with key off, but it should hold above 20psi for MONTHS and MONTHS
If you had a leaking injector you WOULD GET Rich codes and O2 sensor codes
So pressure drop is most likely check valve inside fuel pump
If its hard to start then cycle key on and off 3 times then start engine, the key off and on builds up lost pressure
Low MPG can be a few things, first would be upstream O2 sensors, they need to be changed every 100k miles or 10 years
Old O2s lower MPG, and there would be no codes because O2 is how computer refines air/fuel mix
Second would be larger tires, yes more weight but the bigger deal is that the Odometer is now off, its showing LOWER miles that actually traveled, so if you are using odometer vs gallons used, the math is wrong so result is wrong
You can look on drivers door label to see stock tire size, if its smaller than what you have then odometer is wrong