Only metal foam crumple zones can provide the biggest mpg advantage!
 

IMO the biggest factor affecting mileage is vehicle weight.
Unless you use exotic materials a lighter large-sized vehicle will tend to be fragile.

Thus we need rollcage style rigidity around the cabin. No problem here so far as race homologated rollcages only weight a few hundred pounds, yet they withstand spectacular accidents.

The problem comes in crumple zones.
Even if optimally shaped sheet metal is a poor material for crumple zones: not elastic and heavy.

This is where foam metal will come to play.
They offer exceptional shock absorbance for its light weight and small volume.
For example you can google titanium and aluminum foam crumple zone reasearch.

Thus IMO the mpg futures is in foam metal crumple zones.

Cars have been doing this for 20 years plus. Plastic and foam have been front and back of many cars, including hondas, since the early 90's.

The weight of the crumple zones is not very much.

Foam crumple zones are designed for low speeds.
They offer little protection at 55mph or 110 mph (you are traveling at 55mph and you crash head on with an incoming 55mph vehicle).

I disagree with weight being the most significant mpg factor. Regardless of weight, my mpgs don't fluctuate much. In stop and go driving weight is the supreme factor, but at any sustained speed, aero is king.

That said, I still would like to see weight reduced because vehicles are more peppy and fun to drive the lighter they get.

Drivetrain efficiency is most important. Second is aerodynamic drag. Third most important is weight, and last is all other rolling drag.

weight is THE easiest changed factor in the first, third, and fourth item in your list.

also interesting is you don't have accelerating the car as significant. Must be a pretty nice commute where you never have to accelerate the car at all.

You know the energy needed to accelerate isn't lost, right?

Right -- weight *helps* with coasting, and if you have an EV or hybrid, you can also "get back" some of the invested energy in acceleration. Also, acceleration is much worse for an ICE powered vehicle (they lose about 75% of the energy vs about 10-15%), so for a "city" vehicle you should have an EV and light weight, with aero moving down to third.

Back on topic: a larger crumple zone with lower weight and good aero all are desirable. But if you throw it all away with a low efficiency drivetrain, then lower weight is more important.

As traffic keeps increasing most driving will become stop 'n go.
And I agree with lightweight.

I'll bite. Ignoring the Prius and other hybrids, how are you "saving" that energy? Heating the brakes at the next stop sign?

An object in motion stays in motion.

Coasting is a great way to use the kinetic energy stored by the weight.

Of course, an accelerated objects energy is always lost on earth due to friction- be it aero, rolling, or brakes.

I believe that good drmiller has misunderstood.
We are referring to aluminum foam, and also to titanium foam. Not referring to aluminum PLUS common plastic foam, which indeed has been in use for many years. Foamed metal apparently has significant strength - though it does crumple - and light weight. Apparently deal for - you guessed it - crumple zones.

I googled "aluminum foam crumple zone reasearch"
and "titanium foam crumple zone reasearch".

I found these links, among others:

http://cvxmotion2.wikispaces.com/Jac...ouglasResearch
http://www.materia.nl/575.0.html?&user_material[material_uid]=439&cHash=10e5e231b2
Center for the Science & Engineering of Materials @ Caltech
AUTOMOTIVE INDUSTRY DEVELOPMENT CENTRE :: NEW METAL THAT'S FULL OF HOLES
http://www.google.com/url?sa=t&rct=j...RPEpmAiazfr2rw

I haven't been a big fan of reducing the weight of my Civic. Maybe I should, but I still want to have a passenger seat, real safety glass windows, etc. etc. I'm too thrifty (aka CHEAP) to get carbon fiber hood and other parts. And I like to carry my tools in the car. Otherwise my son has been known to borrow them...

That said, manufacturers are in a good position to reduce the weight of vehicles in the design stage, and they've been doing this for many years. They've also been complying with gradually increasing safety standards, so IMHO most cars have gradually gotten heavier. But if they can reduce weight and get improved crumple zones, I'm all for it.

Moving weight = kinetic energy. Once you have "invested" energy accelerating a vehicle, you can then use that kinetic energy to coast. You lose energy to aero drag and rolling drag -- both of these are a total loss.

This is why aero drag is more important most of the time than weight -- aero drag is a majority of the drag most of the time, and it is a total loss. Kinetic energy can partially used, and if your drivetrain is very efficient and if your drag is low, then weight matters less than if the drivetrain is not efficient and /or the chassis is high drag.

Here is an idea, combine the need for energy absorbing foam with that of storage battery for a hybrid or all electric car.

Carbon foam as I recall it, was developed under DARPA as a heat resistant foam for aircraft using composite construction (my facts here may be fuzzy). It has unusual electrical properties, a great conductor and something about static charges being held (and or attracted) which didn't make it the best thing to fly in during a thunderstorm.

Carbon Foam: The Key Ingredient of a Better Battery?
Carbon Foam: The Key Ingredient of a Better Battery? | Michigan Tech News
EDIT: If anyone knows what happened to the real/original Carbonology company which made T-Foam I'd like to know what happened to them. Looks like another company took their name now.