But most of the time we assume (or have evidence that? -Matt?) petrol demand is fairly inelastic (hence the proposals to vary GST on petrol). With this in mind I was quite intrigued to see this article about Nikki who switched to driving her van when petrol prices soared and has since switched back to her EVO VII now that prices have fallen.
Her demand for being able to drive everywhere is inelastic, but her derived demand for petrol appears to be quite elastic:)