Just as a flywheel stabilizes the output of an engine, I think the mass of a heavy vehicle actually helps it once it starts traveling at highway speed. It carries a lot more momentum, plus the heavy vehicles also have larger displacement engines which barely have to turn to maintain a highway speed of 65 mph. Its simple... An object in motion will want to continue to stay in motion. the engine has to do little to keep it going.

-Jay

easy. I towed a trailer with an F250 2 weeks ago. Going to pick it up: 16.7mpg. Towing it home: 9.2mpg. I had a tailwind going, headwind coming back, so it exaggerates the stats, but the truck weighs about 4700lb empty, and the trailer was pretty close to 3000lbs. In town was just as bad: slower starts, longer braking etc etc. Weight of that magnitude will make a difference.

Its just the 200lbs in a sub 2500lb car with a motor that can handle 3000lbs just fine isn't going to make a huge difference (maybe 1mpg). And at a cost of $500-700 for a hood that's 10-15lbs lighter, that's not worth it unless you can make it pay for itself within a reasonable amount of time.

I will find out in 2 weeks how much of an impact 3 more (over 2) people in my GP (3380# empty) make on the highway. I'm assuming 3-5mpg hit. I know there's not enough lightweight aftermarket parts to offset the weight of 3 people to keep that 3-5mpg. I know that after all the parts swapped, It'd only be about 140lbs saved. To my car, thats a passenger to me driving. It doesn't notice.

Like beef said, it can be done, its just way too expensive for the net gains.

For normal city driving, weight does matter, but you have to talk about a significant amount. 300 pounds in a 2000 pound vehicle is significant. 100 pounds in a 2500 pound vehicle is probably not.

Common hypermiling tactics may make weight significantly less important. When hypermiling, one avoids acceleration as much as possible, trying to coast instead; acceleration is the time when weight matters. Hill climbing is affected, but while descending extra weight can help keep DFCO longer or gain more speed.

How much weight do you plan to save by replacing parts?

another factor about the VX example is that it has lean burn which kicks in and shoots your air fuel ratio to something like 17:1. I am not an expert on this but I have heard about it. that is why the VX owners are...well that is one reason the VX owners are getting stupid good mileage.

I have heard of someone taking 150lbs out of a geo metro and getting better mileage but they didn't replace it with anything. took out the seats, body panels (interior) and insulation (road noise deadening material). personally I want some level of comfort in my drive and versatility in case someone else wants a ride. you can take out your spare tire and save a little but one flat will kill all your profits from gas savings. just my opinion

the hx comes with air conditioning, power steering, electric door locks, and electric windows. depending on the options of a particular the DX it can be had with out power steering, air condirtioning, or electric anything else. I have a 96 DX hatch and the only difference between it and a base CX was the passenger side air bag. it has power nothing.

a lighter vehicle with a drive train identical to a heavier vehicle will get better mileage. it takes more energy to accelerate and maintain the velocity of greater mass. the only exception would be if the route a measurement was taken from was downhill. the word "insignificant" is vastly abused in automotive forums and seldom used by engineers. There are measurable differences between systems- whether or not one considers a difference "significant" is a matter of opinion, economics, convenience...

the vx and hx have different drive trains. the vx is lower powered and more efficient regardless of the weight difference between it and the hx. it also has a smaller frontal area and I believe a lower drag coefficient. taller gearing too...

Don't forget about lightening the parasitic loads on the engine. Aluminum pulleys, carbon fiber driveshafts and lightweight rims.

Maintining velocity of greater mass requires almost zero more energy. The only slight difference is a tiny amount of rolling resistance, which barely differs with more mass (unless we're talking about many times as much mass). Furthermore, additional mass can help a hypermiler have longer glides, as the additional mass beats aerodynamic drag better, and is probably less affected by bumps in the road.

Now, that's not to say that I advocate adding ballast; indeed, if you can remove a LARGE portion of mass without consequence, it's probably still a good idea. The gain from removing the spare tire, jack, and backseat in most cars would easily by beaten by always driving on half a tank instead of filling it up all the way, though.

Thats what i did on my mazda, removed/gutted as much as possible and drive with only 5 gallons. Since i post in the garage i have been filling the tank. I may go back to 5 gallons to save more weight and do aero mods now that i have sort of a base line.

Most carbon fiber products are actually fiberglass products with carbon fiber overlay.

Cost will never be justified. But I would suggest getting a one-piece fiberglass front end. You're wasting your time just doing the hood alone.

It's much cheaper to get the Odyssey motorcycle battery in place of the factory battery. Same weight savings, much much cheaper price.

when I say "maintaining velocity" I mean in real world driving. When I am driving a car with a real time gas millage computer it is very very rare that the mileage settles on some number, say 40 mpg at 60mph. In the western U.S. (where I live), it is very very rare to drive on a flat road. throttle position is changing almost constantly as I respond to: slope, wind, air density, safety, merging, passing, police presence, animals in the road... the computer millage reading reflects those changes. I try to minimize throttle position changes to maintain a constant speed but still: In order to maintain a mass at a given speed I am changing throttle opening in order to do it. It takes less energy to maintain the velocity of less mass than more mass. it is measurable. it makes a real difference.

5 gallons of gas wiegs approc 30 lbs. if you take more than 30lbs out of a car you are allways hauling around 30lbs less mass. you will get measurably better milage.

"Furthermore, additional mass can help a hypermiler have longer glides, as the additional mass beats aerodynamic drag better, and is probably less affected by bumps in the road."

the above sounds like conjecture to me.

a lighter car with all else being equal will always get better millage unless you live in a world where you are always going downhill.