Aerodynamics (what do you recommend, where can I get it, or should I just build it myself)
Intake (Warm air or cold air, what type of filter)
Water Injection (I read a little about it)
Hydrogen Boost (Do any of you know about this?)
PCV Valve catch
Tires (what kind of tires and where to get them)
New plugs are good. NGK properly gapped.
New wires are good too. OEM wires are the best.
Most fuel additives are a waste of time and money.
Oil additives like slick 50 might help. I don't have much experience with that. Hopefully someone else can add their input.
Aerodynamics mods will really help. Undertrays, grille blocks, or anything else you can do to smooth it out are good. Basjoos has a thread about his aerodynamic modifications that are great.
Most people here use lighter oil and have noticed success (0w 20, etc.)
Same for transmission fluid.
I've yet to see any data for water injection for EFI engines.
For hydrogen injection even the commercial products have yet to provide any data to show that it works.
PCV Catch can will help with long term gas mileage by keeping your engine clean.
for tires get LRR tires. They are the tires used on hybrids. LRR means Low Rolling Resistance, so essentially it has less friction on the road. You might want to do like krousdb did and get light rims from a civic HX and put tires from a Honda Insight on them. They are lighter and have lower rolling resistance.
Besides that, I hope other people can give more info. Welcome to the site and we hope to see more of you around!
BTW, 45 MPG is "good." I always laugh when people call 45MPG low.
I am very impressed with your mileage stats. 65 with your VX and 66 with your Del Sol?!? That's insane. I want to get to that point. I will do whatever I can. I have the money and the ambition. Do you have any other mods, besides aero kit, on your VX? What have you done to your Del Sol to get that kind of mileage?
On both cars, as Matt mentioned, I had/have lightweight HX wheels and the OEM Insight tires. On the Del Sol I had a belly pan and grille block. I will have that on the VX as soon as I pass inspection. The key to high mileage for me is low speed and engine off coasting. On my daily commute, it takes 55 minutes to go 25 miles (each way), so ave speed is just north of 30MPH. Secondly, I live in a hilly region, so I plan my route where I can turn off the engine while going downhill and bump start before climbing the next hill. On my round trip commute, my engine is on only about 60% of the distance, 30 of the 50 miles. So that means that I coast for 20 miles each day.
The VX is much more of a highway cruiser than the Del Sol due to the tall gearing and the vtec-e lean burn. I have actually changed my route from 25% highway to 50% highway and still show slightly better results over the Del Sol. My highway driving is in a 55 MPH zone so I just drive the speed limit. At 55MPH, the VX is capable of 64+ MPG, even in on hilly terrain. I expect that flat terrain would be even better. With the belly pan and grille block, I am hoping for 67+ MPG at 55MPH.
stock tires on the vx are 165/70 13" next time I buy tires I'm thinking about 155/80 13" they would be 10mm narrower, and depending on the exact tire, the tread contact patch might be even narrower then that, they are also going to be cheap and light, altho shoping around for as high of presure, and long of tread life as possible is going to be a small investment in time, another option if you can aford it, are the honda Insight rims, they are more of a disk wheel, more aerodinamic, and around the same weight as the HX rims.
the aluminum brake drums from the crx hf made in 1986 (I checked, 1986 really is the first year they did that as a stock part) and latter are lighter weight aluminum with iron lining insted of full heavy iron wheel drums, the insight drums might fit as well, but I've been told that they are not as wide, so you end up with a gap next to the backer plate.
spark plugs, I've had best luck so far with NGK v-power (stock plugs) copper plugs are more conductive/efficent, but don't last as long, platum plugs are "more efficent over the life of the plug" or more so, after that point in time that you should have changed them.
I took out all my injectors one at a time, and cleaned them, they were extreamly dirty, even after useing a fuel system cleaner, altho I don't think that those products hurt.
my grill is compleatly blocked off except for a 6x6" area right in front of the radiator, and with it being 100+ degrees here the last few weeks, the temp has not creaped up to high, replacing your thermostat with a hotter one will increase your fuel mileage, but it will also increase your emmitions output, so they could have made the stock car get better mileage, with a sacrofice to the enviroment by putting a hotter thermostat in, stock is something like 182F or something that seems a bit low.
when your o2 sensor wears out around 200,000 miles your check engien light will most likely not come on, the car will simply run a bit odd, the gas pedal will feel much less sensitive and your mileage will suffer, and that is the time to cash in your retirement plan.
driving has alot to do with your mileage, stay under 70mph, and it will go up a lot, shift at or befor your shift light comes on, and take it easy! when coasting take your foot off the gas, and leave it in gear, as long as the engine is turning over 1,100rpm (?) the fuel injectors are turned off (no fuel being burnt) altho you will slow down a bit, if it's a short coast it can be alot easier then turning the engine off all together.
I'm running full synthetic and feel it makes a differnce, 0W 30, and soon synthetic manual transmition oil.
my mileage is also in the 40-50mpg range, alot of highway driving, alot of it with tools in the car/extra weight, and I agree, 45mpg is low, I cringe if it gets down to 40mpg, because another 10mpg is another 100 miles on the tank of gas.