Lift pump changing fuel mileage

[Shadetreemechanic]

The guy that drives my 85 f-350 says he has moved from 14 to 15.5 mpg in the month since I replaced a failed lift pump. It was mechanical before and is mechanical now. Its a delivery truck and gets about 500 miles per week on the same route.
Any theories as to why the mileage would change? I'm confused.

 

[OLDBULL8]

The only reason for that is that I can think of, would be that the engine is running more efficiently, no air to the injectors.

 

[firehawk]

Drag on the motor from the pump. I am always amazed how a little friction can affect efficiencies.

 

[justinray]

If the previous pump wasn't pushing adecquate pressure, that would advance the timing back and forth when it would surge. Iirc, when I checked, more pressure retards timing and less advances it. Seems like it would be the other way, but I believe that's what I found.