Was asked here several times about improvement results from the belly pan. Not easy to do an A-B-A type test but you can read through my gas log which shows the improvement as it was being built.
Gas log for Lean Burn Civic
Note I had the head rebuilt just before starting the belly pan. First tank after the new head was March 2. So you can review the mpg from that point forward. First couple fills after the rebuild were about 43-46 mpg, then up to a peak of 51-52 mpg with 2 and then 3 sections of the pan installed.
I have to admit, the biggest improvement seems to have come from the front section. The rest of the pan has some odd areas due to the exhaust. I think you'll see that if you read through the gas log notes. I still think the rest of the pan is worth building though.
I removed the belly pan below the muffler somewhere between April 5-12 and haven't gotten around to putting it back in that area. I hope to restore some of the gains by doing that. Something seems to be going on with the engine, not running quite as smooth (I think) as immediatelyl after the head rebuild. I'm sure that's part of why mpg has dropped to about 46-48 lately. All fresh tuneup parts so I'm not sure what the deal is. Nearly 230k miles on this engine!