Explanation:

- If the price of petrol increases by 81%, the cost becomes 181% of the original.

- To keep the expenditure constant, consumption must decrease proportionally.

- Let original price per litre be P, and let original consumption be C litres.

- Original expenditure: $P\times C$.

- New price: $1.81P$.

- New consumption needed: $\text{New Price}\times \text{New Consumption}=\text{Original Expenditure}$.

- Solve: $1.81P\times \text{New Consumption}=P\times C$.

- New consumption: $\text{New Consumption}=\frac{C}{1.81}$.

- Decrease in consumption: $C-\frac{C}{1.81}=\frac{0.81C}{1.81}$.

- Percentage decrease: $\left(\frac{0.81}{1.81}\right)\times 100=44.75\mathrm{\%}$.

Correct Answer: 44.75% (Option 4)