Editing Improving fuel economy

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This is a common forum topic, so it will likely make a useful wiki article. Could potentially divide it up into sections corresponding to automotive systems and practices. 

For now, you can just add tips to the list below:

*Keep tires inflated to proper pressure.
*Keep up with regular maintenance ie. clean air filter, properly gaped and clean spark plugs
*proper alignment

*on a red light shift to neutral while it turns green


==Resources==
*[http://www.fueleconomy.gov/ FuelEconomy.gov]
*[http://en.wikipedia.org/wiki/Fuel_economy Fuel Economy at Wikipedia]
*[http://www.gasbuddy.com/ GasBuddy.com] -- lowest gas price search

A few more little suggestions
1) exercise wisdom when choosing tires.  Wide tires look nice, but present more rolling resistance to the ground.  Use only the width you need.  Also, take a look at the tire manufacturer's website and look at the actual tread width.  The section width listed with the tire size is not indicative of the actual tread width that sees the road.
2) use that same wisdom when selecting those big 22" heavy wheels.  Although wheels won't affect steady state MPG, the added rotating inertia can affect MPG if a lot of speed changes are used.  It takes more right foot input to get big wheels up to speed.
3) another consideration is the tire's rubber compound.  Touring and economy tires usually use harder compounds which sacrifice grip for longevity.  As a very general rule, tread block size is often correlative.  Smaller treadblocks like can be found on touring and utility tires often correspond with harder compounds.  Harder compounds and smaller treadblocks can mean a little extra MPG help.
4) avoid the temptation to remove all the smog equipment in search of MPG.  In many cases they help, especially EGR.  The smog equipment started becoming mainstream in the early 70s, which is also when the SAE standardized dynomometer testing, and also came with heavy EPA mandates on emissions.  The low MPG associated with this era is mistakenly blamed on smog equipment, when in fact it should be blamed on abyssmally low compression and lack of today's technology.  They were neutered muscle car motors.
5) consider diesel in your next car purchase.  Although diesel prices are much higher right now, the associated mileage you can typically achieve almost always more than offsets the cost.  Other benefits are monster torque, resistance to the fuel being ignited in an accident, and the ease of using environmentally-friendly biofuels.  Plus, the resale value alone will usually more than offset any additional maintenance cost.
6) Put a block of wood under your accelerator.  Not kidding.  It really works.
7) for the more adventurous, fold in your mirrors and tape up the gaps around your grille and headlights.  a respected diesel magazine was able to get almost 2 more MPG from a cummins diesel dodge from this modification.
8) consult others who have the same model of car before hacking into the suspension, but most cars can gain a little MPG by finding the right stance.  I picked up a little measurable MPG when I lowered my full-size GM, but it depends on the aerodynamics of your specific vehicle.
9) don't assume the a gear swap or overdrive will help.  Best mileage occurs usually just below the torque peak, at or near peak VE.  Making the RPMs too low can cause lugging, which means you'll be using more fuel trying to maintain speed.  If you have a very high-rpm torque peak, then all bets are off, but chances are you're not worried about mileage anyway :)
10) modern EFI cars are often tuned with a fuel map that keeps things a bit rich.  This is for two reasons; first, to favor NOx emissions at the expense of a little added HC emissions, and second, to allow for a small buffer against detonation.  Experienced reprogrammers (even Jet and Hypertech) use this information.  By pulling a little fuel out of the curve, you can usually pick up 5 hp and sometimes just a little MPG.
11) resist the temptation to use a low-temp thermostat.  True, it can make power, but at the expense of MPG and oil life.  Drivers like their engines to run cool, but engines like to run hot for their health.  Using a 195 stat can often times really make a noticeable difference in MPG, and your healthy oil will thank you.
12) avoid using exhaust crossover blockers.  For the same reason as #11, the hotter the intake air is, the better mileage it can get... of course at the expense of power.
13) Use one of those water HHO converters for 600 mpg.  That's a joke.  Don't.