GM 8 Speed Less Fuel Efficient?

2016 Chevy Silverado 5.3L with Eight-Speed Gets 1 MPG Less Than Six-Speed
GM Touts Benefits of Smoother Shifting, Better Trailer, Load Launch

Quote :

In yesterday’s post of the fuel economy figures of the 2016 Ford F-150, we also stumbled across some puzzling information for the 2016 Chevrolet Silverado 1500. In looking at the official EPA numbers, the highway fuel economy figures for the 2016 2WD model equipped with the 5.3L V-8 and the new 8L90 eight-speed automatic transmission, the highway number came in 1 mpg lower than the six-speed at 22 mpg. This seemed counter-intuitive to us, with conventional wisdom telling us that a higher number of ratios and a wider ratio spread should result in better fuel economy. To double-check that this wasn’t a data entry error, we reached out to General Motors to confirm these figures were correct.

GM confirmed the figures are correct and offered an explanation for the variance. The official explanation is that because the eight-speed is currently offered only on the higher trim level trucks, those trucks have heavier weights than the more basic models equipped with the 5.3L V-8 and six-speed transmission. General Motors cited other benefits of the eight-speed beside fuel economy, including smoother shifting and better launch with a payload and trailer. GM also said it expected real-world fuel economy on comparably equipped models to be slightly better with the eight-speed. For reference, the fuel economy for the 3.5L Ford F-150 EcoBoost 2WD with the 7,599 GVWR also came in at an official EPA rating of 16 city, 22 highway, same as the Silverado 5.3L with the eight-speed.

Truck Trend

I wonder if the top gear may be too high and the engine has to struggle just a little to keep speed?

Or engineering decisions that favored performance over efficiency by a small margin.

That's odd. Hopefully the 10sp isn't 2 mpg lower.