How to make a car more fuel efficient?

I thought fuel cells use thermodynamically open system to produce electricity. A Magnesium-Air fuel cell can achieve 90% efficiency.

I meant fuel to electricity cycle using alternate means rather than internal combustion engine....
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I thought fuel cells use thermodynamically open system to produce electricity. A Magnesium-Air fuel cell can achieve 90% efficiency.

I meant fuel to electricity cycle using alternate means rather than internal combustion engine....
Yes, theoretically shaft horse power, chemical energy, photons and other EM energy, gravitational potential etc (all forms of energy EXCEPT HEAT) can be converted into any other high grade form of energy with 100% efficiency. It is only heat conversion to any form of high grade energy that is Carnot limited. Heat its self comes in different quality grades (higher the temperature the higher the quality). When part of the thermal energy is converted into a higher form, the remaining thermal energy is converted into lower quality heat.
Collect and use that, you can increase efficiency. Best solution is to convert fuel to heat and heat to electricity to get 90% efficiency.
Both these sentences are false.

The first as there will be some energy loss in your "collection process" - probably at least the temperature drop thru some heat exchanger. Thus even if you had a thermal engine that worked with Carnot efficiency you would not be able to do much.- very probably more poorly than simple dropping the exhaust Tc and getting increased piston pressure as it descends with the water mist injection I suggested.

The sec one is false as you let the high quality chemical energy in the fuel convert into the low quality thermal energy and then can only recover the Carnot fraction back, at most (in practice much less) as high quality electrical energy.
Make the back end of your vehicle tapered more, instead of square as most are.

Lose weight of course, and keep higher pressure in your tires.

A neighbor of mine found that preheating the incoming gas a bit, by taking an insulated turn with the fuel line around the radiator hose, boosted his winter gas mileage significantly - no effect in summer.

If you have a pickup without a topper, replace the tailgate with a mesh panel, one of those rubber ones you see is fine (better than empty space!). With a topper try to rig a taper in back, or get some air flow into the hollow area behind - you don't want to be towing a huge low pressure area around, or its associated vortices.

If you have the means, there are some tricks with grooving the cylinder head and polishing the ports, etc.

Synthetic oil improves my gas mileage enough to pay for itself.

Most people could get a big improvement just by keeping their tires inflated properly - and that's both free and easy.
Perhaps I should have said "Best solution is to convert fuel to electricity to get 90% efficiency." ?
Are you sure? In industrial plants, the latest technology is waste heat boilers to get additional work from a combustion process.
Certainly the second sentence was false for reasons already explained. The first is false I think, if it is understood, as I did, to be saying that rather than get more work out of the fuel by water mist spray injection, it is better to NOT internally cool the combustion gases, but let them escape and be collected to run some low temperature difference, L.T.D. thermal engine

The water spray gets more work out of the fuel's heat WITHOUT TRANSFERING energy thru any input heat exchanger. Likewise, additional losses would be incurred as the waste heat of your external (to the IC engine of the car) low temperature difference thermal engine is dumped thru a second heat exchanger. Cne should also note that this external L.T.D. engine has weight that the car must transport. I doubt that the energy recovered from the exhaust will even compenstate for the losses associated with that weight in a car. You probably can at least "break even" in a big heavy truck.
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Make it as light as possible. Don't cut the frame or outer panels, just remove or unbolt everything that is not needed. Tear out sound deadening material. Remove the passenger seats. Fashion fairings to cover the rear wheel openings. Attach a flat sheet of plastic underneath the car. Create an airdam in the front, very low to the ground. Cover most of your radiator, you don't need it all. Inflate your tires to the maximum listed on the tire. Disconnect air conditioner. Remove outer mirrors and replace with ones behind the window. Use a lighter weight oil.

I was pondering something like that a while back. I saw all the youtube videos of the Ariel Atom and thought "what if you made something like that except with a normal engine?". Basically a skeleton frame with wheels, suspension, indicator lights, engine, etc......but not designed like a sports car.
About the OP,

1. Reducing wheel size increases rolling resistance. The gauge pressure increase masks this effect, but if one were to apply the same pressure increase in a regular-sized tire, the footprint would be the same with slightly less rolling resistance (also, there would be a longer tread life in larger tires). But what does it matter - rolling resistance pales in comparison to air resistance above 30 mph (most speeds traveled) for a car.

2. The most effective things are to eco-drive (take out unnecessary seats, weights, etc.) and to check the air filters regularly.

3. You can also buy aluminum wheels, but that has again a small impact.
2. The most effective things are to eco-drive (take out unnecessary seats, weights, etc.)
This isn't the first time I've seen that advice, but given that your car is probably at least 1500 kg or so, I'm a bit skeptical that removing seats etc. would have any significant impact. The seats might each weight 25 kg or so, but that's still going to be a relatively trivial weight reduction.
I've always wondered about the seats myself. I'm getting my info from an article published on one of those environmental periodicals, either Mother Jones or Sierra magazine, back in 2006 or 2007. The experienced hyper-miler took out all of his seats.

I suppose it does make some sense, if all the seats are taken out. Instead of 1500 kg, it can be 3x25kg = 75 kg weight reduction. Over the long term the effect is noticeable, but maybe in the short term it has little noticeable effect.

The aerodynamics have a lot to do with the efficiency. Tailgating a tractor-trailer can be dangerous, but it has noticeable benefits. I dare not to get that close, however.

There was another strategy, and that was to coast on neutral for downhills. The coasting technique also applies to flat terrain, where one accelerates to a certain speed above 60 mph, and coasts until about 45 - 50 mph. However, I think it has only marginal advantage since newer engines are specifically designed for efficiency not at high rpms, but at lower rpms when someone cruises on the highway.

The timing of the spark plugs on older cars can be adjusted to get about 45 mpg, examples include a Datsun that my dad modified. Instead of conservatively firing on the down stroke, the spark ignites nearer to the point of maximum compression - hence more distance traveled by the piston, or more work done for the same gas expenditure.
This thread may be a bit cold now, but as no one seems to have pointed it out, I will.

To answer this question properly, you first need to ask a second question: What kind of driving do you most commonly do?

So, if your around town/in traffic at low speeds constantly starting and stopping, the ways to make your car more efficient (we'll assume this question - as most posters have taken it - to mean improving efficiency as an after-market exercise, ie. not at design stage) then it is going to be quite a different exercise max. your efficiency than if you drive out of your street and spend an hour's commute on the highway at 70mph.

I am also doubtful about the anecdotal evidence for water-mist injection (and even more so for so-called Brown's gas BS!)... I would hazard a guess that the supporters of these techniques are running older (probably carbureted) vehicles with already quite poor BSFC due in part to poor in-cylinder burning and over-protective timing setup.

One easier way on a more modern vehicle (if your not concerned about air-quality) is to dump the catalytic-converter - ideally with a remap for cooler exhaust temp.
Reducing or even eliminating losses due to cooling system
and losses due to flue gas.
Make it ligher. A bicycle and rider get the equivalent of 650 mpg. Although the fuel cycle is much more efficient than an internal combustion engine, the coefficient of friction isn't all that great. But when you put a decent faring around bike and rider, particularly if it's a recumbant, the mpg is approximately tripled.

So consider that an upper limit, and also consider why: A modern, high-tech bicycle weighs 10 to 12 pounds with tire pressures arond 120 psi. Thus, there's absolutely minimal rolling friction.

And bicycles with a small gas motor get up to 225 mpg! That's about 60 lbs, not including rider, at 30mph.
I believe we cannot do much about the combustion process, which is root of inefficiency. What we can do is harvest the energy that is being wasted, like today I was in Toyota's office and a person there said why dont we collect exhaust gases in a cylinder inside car and use them for household purposes ?

BTW once I had thought of having cylinders for each wheel separately rather than having a same engine placed far from each ones :D
So many ideas...
People haven't looked at steam in forever.

Very powerful! These vehicles usually only had one gear. Incredible torque but very low horse power.
These cars were comparable to modern full size vehicles, in terms of weight, weighing in at about 4000 lbs. (2 tons).
In 1906, a purpose built steampowered car ran about 127 MPH. The vehicle weighed about 1,675 lbs. and it kept the world record for 5 years.
My high tech, very streamlined, stock 1999 Miata will only do 123 MPH and it's about 2,000 lbs.

So, what about today?
What would a modern steam engine consist of and how efficient on water would it be?
How about a two cylinder, turbo diesel, electric, steam hybrid?
Instead of electric motivating the vehicle, use steam, but use electric for everything else as in a prius or other electric vehicle.
This way you still get energy reclamation from the electric side, even while the vehicle is running on steam power.
Of course, there are lots of questions that need answers.
Primarily, electricity would heat the water.
What would be the most efficient method and materials?
How do you prevent water loss?
What is the minimum amount of water you would need?
Would a direct connect from the motor to the differential be best?
How about an ECVT?

A GM EV1 seems like a good starting point. 0.19 coefficient of drag.

What do you think?