My car costs me about the same as my groceries over the same period of time; they are very comparable in price. As for sales - buying in season and buying in bulk saves huge amounts of money, a larger %age than I expect to get off a special deal on a car.

For groceries that don't keep, you are obliged to buy when you need them, although because humans are omnivores you _can_ choose to eat steak only when steak is cheap or eat raspberries only when raspberries are cheap and save quite a bit that way.

However most peoples' grocery shopping includes staples that have a relatively long shelf life if stored sensibly. Those who aren't dirt poor can and should purchase larger volumes of these products at lower prices if they know lower prices are not always available.

This is one of many ways that being poor is expensive, your financial status makes it impossible to invest up front on bulk items that would be cheaper over the medium term.