You have to also account for the free 1/8. Really enjoyed returning all my cars at 7/8 and them considering it returned full :). Part of it is most fuel gauges still read full anyway.
actually, I nearly always buy the tank because my time is worth more than <$x>, it's one less thing to stress about, and the refill prices are really punitive and when it's on your dime, they top it up.
sales dude: you want to prepay for that full tank of gas, super cheap only for you my friend, just today
me: no
sales: if it's not full it's going to cost 50% more
me: no
sales: look at the gas station prices
me: I never return with completely empty tank so whatever savings get wasted on the fuel I gift you on return
sales: but it's so much cheaper
:%s/gas/electricity/g