Scott Hamilton picked up a few interesting details from the Boeing Quarterly Earnings Call regarding the B737RE. The most important question regarding this program remains the fan diameter. It looks like it will settle around 66 inches, so that Boeing can go forward without touching the nose gear.
But what does that mean for fuel efficiency?
In another post I tried to make some assumptions based on the front fan area and fan pressure ratio. Meanwhile I have some other data, showing the effects of fan pressure ratio and fan diameter on SFC and fuel burn. I cannot reveal any numbers here, but basically one inch in fan diameter is worth about 0.4% in SFC. So if a fan diameter of 71" was deemed as the optimal diameter, a 66" engine would be about 2% worse in SFC.
Using some other data I have I can conclude that the LEAP engine for the B737RE can be about 11% better in SFC than the CFM56-7B (or about 10% better than the -7BE).
What does that mean for fuel burn? For a typical mission profile of a single aisle aircraft one would say that 1% in SFC is worth 1.05% in fuel burn (as your reengined aircaft is lighter, it does need less thrust, saving more than the obvious 1% in fuel burn).
But the engine is larger and heavier. The fan diameter is 5 inches larger than today - let's assume CFM can cut 1 inch in nacelle thickness, so the nacelle is only 4 inches wider. The larger drag of the nacelle is produces around 15lbf of additional drag in cruise conditions - this is equivalent to about 0.2% fuel burn.
We don't know how much heavier the engines will be than today's CFM56-7B - the neo engines are said to be about 400kg or 880lbs heavier than the V2500/CFM56-5B. So let's assume that the B737RE LEAP is at least 440lbs heavier: this is worth about 0.25% in fuel burn.
There is also some weight added to the airframe, as the engine pylon, the wing and the wingbox need to be strengthened: 1000lbs is a fair assumption at this point, eating another 0.625% in fuel burn.
Bottom line: 10% fuel burn improvement over the B737NG is what future B737RE operators can assume. To get more out of the reengining will be a big challenge for Boeing...