2014 bmw 328d diesel

Alex Davies / Business Insider

After 40,000 miles, the BMW 328d diesel becomes a good deal.

One of the big stories in the auto industry this year has been the growth of diesel sales in the United States. No longer dirty and smelly, diesels are a great way for automakers to meet federal corporate average fuel economy (CAFE) standards.

That's because cars powered by diesel are significantly more efficient than those with gasoline in the tank, especially on the highway.

But diesel fuel is more expensive than gasoline (thanks to dumb fiscal policy dating back to the '80s), and the cars themselves are usually more expensive by a few thousand dollars.

So how many miles does it take in a new diesel to see the savings at the pump make up for those extra costs?

A lot, it turns out.

Using EPA-rated combined fuel economy numbers, base prices, and the national average price for regular gas, premium gas, and diesel (from AAA, for January 9), we compared nine pairs of diesel and gasoline cars. Apart from the engine, there aren't significant differences between the cars in each pair.

Here's the chart. You can see the base price difference for each, how much money using diesel saves per mile, and how many miles driven make up for the extra cost.

We've noted cars that use premium gasoline in red, and the most cost-effective diesel model in green. Cars are model year 2014 unless otherwise noted. Click to enlarge:

diesel vs gasoline car costs

Alex Davies & Andy Kiersz / Business Insider

It's not surprising that the break even mileage number is the lowest for the luxury cars — premium gasoline is closer to the price of diesel. And the markup for a diesel engine has less impact when the gasoline version is worth over $30,000.

Based on our math, the Chevy Cruze Diesel and VW Jetta TDI will never break even, because their fuel economy numbers aren't good enough compared to the gas version to make driving on diesel more efficient.

The best option is BMW's 328d, a car we recently reviewed and liked a lot.

So if you're looking to save money with a compact car, diesel isn't the way to go. If you want to spend less time at the pump and feel the savings before you put 100,000 miles on the odometer, BMW has some cars that should interest you.

Thanks to reader Andrew Maddox for suggesting this idea.