If you have a fixed monthly budget for fuel and the price of petrol goes up, you have to drive fewer kilometers to spend the exact same amount.

Up by $\frac{1}{n}$, Down by $\frac{1}{n+1}

Expenditure is the product of price and consumption ($E = P \times C$). Since expenditure remains constant, price and consumption are inversely proportional. When price increases by a factor of $\frac{1}{1+x}$, consumption must decrease by $\frac{x}{1+x}.

Remember the shortcut: if price increases by $r\%$, required percentage decrease in consumption is $\frac{r}{100+r} \times 100\%$.