I remember my dad doing this when I was a kid, too. It used to infuriate me, because I did the calculations and thought there was no point. He'd also work out the fuel economy of the car to two decimal places, using the formula (miles/litres)x 4.546; not 4.5, but 4.546. I remember trying to tell him about evaporation, errors in measurement of the pump, engine efficiency and so on meaning that you can't get a great degree of accuracy, but he was having none of it. I remember one week, the car had done 40.17 mpg instead of 40.21 or something and he was furious.
(Sutcliffe's Ball Pein, Wed 25 Jun 2014, 17:15, Reply)