Fuel Efficiency differences 870 vs 871

[Chamberpains]

Air actuated turbos compared to electronic controlled is huge plus the Truck design plays a part. The 870's were put in the starts of the aero-cab style's. But a lot of them were inside old heavy box style rigs.
I also believe that the ECM is more complex and able to adjust more on the 871.

I think it's a case of where they are almost identical but it's a lot of little changes piled on top of each other that makes a big difference in the end.