Not to be rude, but why even worry about this?
If you can afford to own, insure and run an "extra" car, you are hardly in a position for this to matter. You'll put less fuel in it than your daily driver due to less time/miles.
Inflation and gas prices are currently up.
It's $5.79 a U.S. gallon in London currently for regular gas.
If you put in 6-7 gallons, you paid about half that per gallon.