It's only a matter of time before rising diesel prices show up in the grocery aisle

The average price of a gallon of diesel just passed $6 for the first time ever late last week. And since then, it’s just kept climbing. On Wednesday morning, it hit $6.31 a gallon.

A year ago, meanwhile, it was well under $4. That, of course, has implications for all sorts of other prices across the economy, too — particularly at the grocery store.

Most of the food you buy at the grocery store spends at least some time on a truck. And chances are that truck runs on diesel.

“And so as the price of that transportation increases, that's going to cause our gallon of milk, our loaf of bread, to increase in price,” said Matt Smith, director of commodity research at the data and analytics firm Kpler.

That pass-through doesn’t happen immediately. “It’s going to be like boiling a frog, basically. It's going to be gradually working its way into prices.”

How gradually will vary by product, per David Ortega, a food economist and professor at Michigan State University.

“Things like fresh produce, dairy, meat, items that have to travel longer distances, that have a short shelf life — meaning that they have to be restocked pretty frequently — you're going to see those costs show up much more quickly,” he said. That could be as soon as a couple of weeks to a couple of months.

“And the more durable goods, the packaged goods, those are going to take a little bit longer before we see these higher diesel prices translate into higher costs on the checkout line,” Ortega said.

The spike in diesel prices is unlikely to push food costs dramatically higher, Ortega added; transportation accounts for less than 10% of what you pay for a loaf of bread or an apple at the store.

But even relatively small increases in food prices can be significant, said Bernhard Dalheimer, assistant professor of macroeconomics and trade at the Department of Agricultural Economics at Purdue University.

“People on the lower end of the income distribution feel these price increases much stronger than people on the higher end of the income distribution,” he said — especially after the last five-plus years of inflation.