Sorry Bleu, I was using the figure of 8.9 gallons per year quoted in the Slate article, which is 33.7 liters. That's how I reached the cost of 27 riyals.

What we can agree on is that it is an insignificant amount compared to the annual expenditure on petrol.

If you want to ACTUALLY save petrol and reduce carbon emissions in a MEANINGFUL way, then drive less and use public transport.