If we're talking about Walmart vs a non-chain supermarket, 5% is going to be about the best possible case. Things like bulk goods or cheap cuts of meat can pretty easily be 30% cheaper. Even looking at a pickup truck and the higher European prices, I don't think a 30 mile drive would rise by more than ~$6? That might still persuade people to make fewer trips and buy more food at a time, though. Which doesn't save local grocery stores, is still a pretty good outcome - cheap grocery bills with less gas burned, and decisions that are closer to reflecting the overall cost of driving.

There's a reason people who live "10 miles from anything" absolutely hate rising gas prices, though. If you commute 20 miles a day for $25k/year, an extra $3/gallon would eat about a week's pay just getting to and from work.