I've been doing some work since joining this forum a couple months ago and perusing the information here. I've gone from consistent 37-38mpg tanks to a best of 51.7mpg and 90-day average of 47mpg! Most of my driving is on the highway, with a 45-mile (one-way) commute three days a week through pancake-flat central Illinois, so I've been focusing on aerodynamic modifications for the open road and gliding and coasting around town. I figured I'd start a build thread here to chronicle the mods and my results.
First up: lowering. I've had height-adjustable coilovers on the Civic since last year, so I lowered the ride height a bit more to get rid of as much wheelwell gap as possible:
Next up were the mirrors. I was surprised how much removing the side mirrors changed the look of the car. Before:
(I ditched the front plate, too). The mirrors were replaced with convex mirrors from Autozone. I didn't trust the 3M tape to hold them to the sail panels, so I used some screws to mechanically attach them:
The driver side mirror started sagging after a few days, so I epoxied the ball joint at the base of each mirror. On the outside, I cut a closeout panel out of some flashing and reused the rubber moisture barrier from the mirrors and three bolts through the stock mounting holes, with silicon caulk to seal the edge gaps.
When I bought the car a couple of years ago, it was riding on the original tires. I was planning on replacing them this spring anyway, so I picked up some LRR Continentals on cheap steel wheels and a set of solid wheel covers.
Finally, to finish the front end, grill blocks were in order. After driving around with a temporary (cardboard and duct tape!) full upper grill block and partial (2/3 covered) lower grill block, I decided to use sheet metal to smooth over the whole front and cover the fog light indentations as well.
The radiator inlet is now significantly smaller, so I've been keeping a close eye on coolant temperature. It seems fine for now, but if I need to enlarge it when 90-degree temperatures roll around, it should be pretty easy to do with a pair of snips.
As it sits now:
Next up, I'll turn my attention to the underside and rear end. My goal for the near future is an 845-mile trip to New Jersey this summer for my brother's wedding: I'd like to do it on one tank (13.2 gallons) or as close to that as possible, which will require the car to get ~65mpg. I'm not sure if that will be possible, but, hey--dream big, right?