In this case, the salesman is right. Unless you're totally smitten with the diesel, go for the petrol. The difference between the cost of a litre of petrol and the cost of a litre of diesel is currently as little as 1p per litre. In some places there is parity. However, the diesel Yaris is much more expensive to buy than the petrol version, which means it'll take you about 56,000 miles before you break even. If the difference between the cost of petrol and diesel widens (it was around 12p per litre 18 months ago), it will take even longer. Small diesels in small cars rarely make economic sense because the petrol units are very good, such as the 1.33 VVT-i in the Yaris.