Hey another hx in dfw! Do you have a factory radio?
I have a JDM d15b trans with shorter gearing and I have been getting better mileage than my last hx (with the factory trans) in the city for sure. The gearing difference is not noticeable enough to justify switching unless your old trans is getting crunchy. As mentioned nearly all gears are the same except final drive but if you have it go for it. My car tends to "bog" under 2000 rpms and at highway I want to be at peak efficiency for acceleration and coasting which seems to be at 2400 for my vehicle at least. As mentioned above lower rpms can mean little because the engine needs to make more power and it would not be in its power curve to efficiently accelerate without lots of air and fuel. My shift points are also as mentioned above and the throttle is much more crisp and response at those levels
I dig the front lip it is a civic vti or si front lip, I went cheap with the Mugen style. I'd like to scoop up some used eibach lowering springs eventually
What kind of shape is your exhaust manifold in? If its original your cat may not be as efficient as it use to be with that mileage and the manifolds known cracking issues. My car has an aftermarket intake
I can't find a stock one anywhere however the radiator fan blows hot air directly into the aftermarket intake which has been having a positive effect on my gas mileage. The intake has also made the pedal feel more responsive. The factory intake is routed to pull cold air in from outside the engine bay which is good for power but not gas mileage
My factory cat was replaced around 220k with the amazon unit and previous owner did mentioned it did improve his gas mileage slightly
My car was on its original plugs with a deteriorated gap oF twice what spec is. You should definitely now do the wires and cap and rotor they look original.
The egr valve cleaning is absolutely necessary. At 294k my passages on the intake were completely clogged, and my valve was stuck shut until I completely dis assembled it and got it working again. Just get a ton of brake cleaner like 3 cans, and clean out the passage on the intake manifold and the feeder passage coming into the egr, and the egr itself. I should mention even online the egr for these cars are still $150, though I do believe an egr valve from the 1998 accord will interchange but I have yet to test that
For the hood hinge you should not need it. Even with my ac on this summer my temperatures barely tip half way on the needle and the fan kicks on only when I've been stopped at a light for a few minutes. You'll be able to block the grille without any ill effects, you should also be able to run a medium sized block in the middle of the bumper behind the license plate. You can try it out and report us back on if it helps aero, but it's basically going to be sucking air out of the engine bay disturbing air flow over the windshield.
I am able to see 34mpg city without much effort. If you are due for tires do not cheap out, get low rolling resistance tires. It really helped my numbers. Get yourself a full tune up and you will be hitting 38mpg with mixed city and highway driving. When I was running the Walmart Douglas tires the best I could get was 36mpg all highway miles, now on my low resistance tires I can get that without using my ac in the city and turning the car off and gliding to a stop light. If I'm lucky I can decelerate enough to just pop the clutch into 3rd gear to restat the car and keep on driving