The fuel economy on some redesigned Chevrolet Silverado and GMC Sierra models will be worse than the outgoing pickups they're replacing.

Ratings on fueleconomy.gov have 2019 models equipped with 4.3-liter V-6 and 5.3-liter V-8 engines getting up to 3 mpg combined less than comparable 2018 pickups. The engines are mated to a six-speed automatic transmission.

It's unusual for outgoing models with essentially carryover powertrains — aside from small changes such as tuning to engines and transmissions — to be rated higher than redesigned vehicles that are replacing them.

Monte Doran, a Chevrolet spokesman, attributed the difference in fuel economy to the redesigned trucks being more capable and larger than the outgoing models, which in turn increases aerodynamic drag.

"We increased towing capacity, payload, and it's a much larger bed and a much larger cab," he said.

While aerodynamic efficiency increased 7 percent and the automaker cut out hundreds of pounds on the redesigned Silverado, Chevrolet says the frontal area also increased — resulting in the same aerodynamic load. The Sierra is slightly more aerodynamic than the Silverado. However, it's still larger than the outgoing model.

Aside from differences in engineering and design, auto writer Bozi Tatarevic, who initially tweeted the differences, pointed out the rear axle gear ratios also are different between the model years.

GM was able to offset much of the aerodynamic drag for models with other engine options, which experienced small increases in fuel economy, with new technologies such as dynamic fuel management and higher-speed transmissions.

The 4.3-liter V-6 and 5.3-liter V-8 engines are standard or available on entry-level and lower-end trims on the pickups.

The 2019 two-wheel-drive models with the 4.3-liter V-6 engine are rated at 17 combined (16 city/21 highway). That's down 3 mpg compared with 2018 models at 20 combined (18 city/24 highway).

Redesigned four-wheel-drive models with the V-6 engine are rated at 17 combined (15 city/20 highway), down 2 mpg from the 2018 pickups that were rated at 19 combined (17 city/22 highway).

The 5.3-liter models with four-wheel-drive and two-wheel-drive also are rated 17 mpg combined. However, the highway fuel economy for four-wheel-drive models is 1 mpg less at 20 highway, and city is 15. That compares with 2018 four-wheel-drive models at 18 mpg combined (16 city/20 highway) and two-wheel-drive models at 19 mpg combined (16 city/23 highway).

The Chevrolet Trail Boss models, new lifted trims with four-wheel-drive and off-road equipment, with the engines are rated at 16 combined (14 city/18 highway).