It was a kinda interesting “test” to see that I got about 2 MPG less on the trip I made when the EGR was not flowing because the EGR bellows was blown out. I had made this same trip dozens of times before.

EGR was not invented to reduce NOX.

EGR was invented in the 1920s to extend the range of gasoline engined aircraft.
The extra inert gas in the intake manifold raises manifold pressure when at less than full throttle power, like when a Bomber is at high altitude cruise using less than full horsepower needed at takeoff with a full load of bombs. The higher manifold pressure “helps” push the pistons down on the intake stroke. The lower peak combustion temperature also lets less heat leak away through the cylinder walls and cylinder head. For awhile EGR was a “secret military technology.”

It was only later in the early 1970s that engineers seeking new pollution reducing techniques realized that the lower peak combustion temperatures led to less chemical NOx creation.