Until an automaker comes along with something better, your cheapest bet for highway fuel economy in a pickup is the Duramax diesel-powered Chevrolet Colorado and its GMC Canyon twin. The full-size Ford F-150 with 3.0-liter diesel V6 matches it in economy, but not price.

Boasting a 30 mpg EPA rating for highway consumption, the oil-burning midsizers command a premium over their lesser siblings, but make up for it with thriftiness and heaps of torque. The 2.8-liter inline-four generates 369 lb-ft of twist - far more grunt than the 275 lb-ft on offer from GM's 3.6-liter V6.

However, there's a mystery afoot. The EPA ratings for the newest Colorado and Canyon diesels show a drop in city and combined efficiency for the 2019 model year, despite the powertrains being a carry-over.

As not all 2019 Colorado and Canyon models have received a rating from the EPA, we only have the four-cylinder gas models and the rear-drive diesel to go on. (H/T to Bozi Tatarevic, by the way.)

The 2.5-liter models see the same rating as last year, but the 4×2 diesel, equipped with the same 2.8-liter and six-speed automatic the 2018 models (to the best of our knowledge), sees its fuel consumption drop by 2 mpg in the city and combined cycle.

Whereas the 2018 4×2 diesel carried a rating of 22 mpg city, 30 mpg highway, and 25 mpg combined, the EPA rates the 2019 model at 20 mpg/30 mpg/23 mpg. The 2019 Canyon diesel, of which only the 4×2 version carries a rating, sees the same change. What's the deal? And why the 2 mpg drop in combined economy while the highway figure stays the same?

We're not aware of any changes to gearing, axle ratio, or engine tune between the two model years, making the EPA's rating a head-scratcher. After reaching out to General Motors on Monday, we were told the automaker was looking into the issue, and would get back to us when it had more information. So far, no word, but we hold out hope for an explanation. We'll deliver that when it arrives.

In the meantime, there's still no official word on the fuel economy of two thrifty new GM engines: the 3.0-liter diesel inline-six bound for the 2019 Chevrolet Silverado and GMC Sierra, and the turbocharged 2.7-liter "Tripower" inline-four bound for the same models. It's GM's hope that Ford's light-duty diesel takes a backseat to the General's effort.

a version of this article first appeared on thetruthaboutcars.com