I have thought about swiss cheesing my car but i don't want to weaken the frame. I use my car for estimations and carrying light equipment. I have gutted it and it seams to help with fe. I have been using 2nd to start then to 4th, skipping 1st and 3rd. I am on my way for my best tank yet. Was also thinking about putting solid motor mounts....anyway its fun to see just what i can squeeze out of my Mazda.

All this is interesting, but...I'd rather have my spare tire, jack, tools, 80-40-20-15 meter CW transceiver, 2 meter radio, several days worth of canned food, several days worth of water, and a couple changes of clothing in the car than not. Carrying all that extra stuff, I am still getting (as of the date I am writing this) 40.57 mpg average over the last three months. So far, modification of driving habits seems to be the best way to increase mileage. Sure, my Auto Club card weighs a lot less. However, when I was growing up, a friend of my brothers' (one of those 'extra' kids every family has) was a tow truck driver. He would tell stories about things he'd do to able-bodied men who called him to fix a tire. Sometimes ended up with somebody being hurt. And it was never him.

I often carry a passenger who weighs 350 lbs. I've never seen a difference in mileage with or without this passenger.

I've plugged sidewalls and driven on those tires for years afterward!

Of course, nowadays I just get the road-hazard insurance...usually about five bucks a tire...

OMFG...just remembered the time a couple years ago driving down the freeway and hearing a BANG! from the trunk of the car...yep, the SPARE blew out! This was about 2005 or so, a 1984 car and I believe it was the original spare...

In theory it does increase mileage, even if it doesn't seem like it and it's a very very small amount. all I hear is how x modification only increases mileage by .5 or something percent. but a combination of a bunch of small stuff like this adds up. with the addition of a multitude of driving techniques.

like said earlier about the cost of a modification, verse how long you'd have to drive the car to actually be saving money is something to really think about. I just bought a bike for $500 and now I have to put another 200 into it to get it running, but It'll still take over a year to save money on gas. unless your an eco modder :-)

but on the subject:
lose weight yourself
fix a flat
all random stuff in the car
I'm not into going to extremes like I was in the past like taking out interior and such. but a lot of parts could be removed or replaced with lighter parts from other models and still not significantly compromise comfort. I know that older hondas have a brake drum that lighter then the other and there's doors that are lighter then others without the seat belt in the door panel. power steering and a/c then switching to the crank pulley without those accessories. ligher battery, but again to see money saved on that would take a while.

I would think that reducing weight would help on hilly terrain; whatever "techniques" you use, you will still need less engine power to move less weight uphill.

a cf hood will save around 14-16lbs which is the same savings as a lightweight battery, this of course depends on brand in both cases.

lightweight fenders are not much lighter. That aluminum is pressed pretty thin. Also the passenger seat is more like 30lbs. if you are taking out all the seats, you might as well take out the seatbelts which total just under 15lbs.

My all time favorite is the bumper supports because they are on the outmost edge (front and rear) of the car. this improves the cars stability as well as shaving off 20lbs.

i have found that less weight doesnt really help mpg that much. it does help acceleration though.

I think that kinda evens out as you have more weight pulling you down the other side...

-Jay

But if you DFCO you really don't use any gas downhill anyway. I might do some tinkering later if time permits. See what it does.

But, if you have more weight behind that DFCO, you have more speed at the bottom of the hill, and more inertia to carry you up the next hill...

When it's all said and done, if you have a net elevation gain (your destination is higher), removing weight will probably help; if you have a net elevation loss (your destination is lower), adding weight may help; and if your destination is the same as your origination (you're eventually going to go back home) then trying to adjust your weight probably won't matter.