Calculate volume and distance between the past twenty or fifty full tank fullups and then calculate distance ÷ volume for each of those pairs. If the number has dropped significantly, you’re getting fewer miles per gallon. If it’s dropped by a few percent starting five or ten days ago, it may be ethanol content (see E15 threads). If it’s dropped by more than ten percent, and has stayed there or worsened over multiple fillups, you may have a failing battery (which fucks up lots of car engine sensors) or a dirty/failing “MAF / AFR / front oxygen” sensor (which causes the car to mistakenly use too much gas). If you’re a thousand feet or more above sea level and your car has started taking longer to crank before starting at the same time, you may have a failing battery (see above) or a failing MAP sensor (which could cause the car to use too much fuel for your altitude).