rear diff low oil effect gas miles ?

[MetalSlug]

I been having bad gas miles since I bought this car, and it been leak pinion seal on rear diff for a long time, and last time I change the rear diff oil, it was low, so I fill it up. anyway since other day I fix the pinion seal, and fill it up with lucas 80w90 with ford friction mod, car running extremely smooth and quiet, I note my gas gauge dint go down as fast as before. so low on gear oil can affect gas miles ?

 

[hisilver]

I would guess an overfull diff would give too much drag and decrease gas mileage. An underfull one would increase mileage up to a point when it gets overly hot and may give more drag?

 

[mechtech2]

A bit low on rear end fluid should give you a microscopic increase in fuel use.
Real low would show increasing fuel use, but that would also be where actual wear was taking place.