Mythbusters already busted this one; even if you add enough PSI to make the tire dangerous, the MPG gain is negligible.
Even if that is true
having the proper tire pressure or more still
makes your car a lot safer, so do it anyway.
Going up from recommended pressure might have little effect, but a too low pressure will definitely increase fuel consumption.
Also if you put in more air it will last longer until you have to re-fill.
My tips:
Don't use the car for short trips. A cold engine will use like two times the fuel on the first half mile.
Buy a lightweight car.
Every time you accelerate you need to drag all those tons up to speed and every time you get a red light all that energy is GONE.
In germany this would be "Buy a lightweight DIESEL car." because diesel engines need A LOT less fuel, but I see they aren't popular in the US.
Drive in a smart way. Don't accelerate when you know you'll have to brake very soon. Every acceleration burns a lot of gas. If you have a red light, let the car roll.
Story: I was in a cab some time ago and the driver was going like an idiot (this story was not the worst he did). One time we were going at a good speed and had a red traffic light approaching in front. It was already red for some time so it was clear (small town and you know your traffic lights) that it would turn green soon.
The driver kept his foot on the gas pedal, maintained his speed and had to brake quite hard to a complete halt. Half a second after we stopped the light turned green.
Had he instead let the car roll 40 meters before the light he would never have had to even touch the brake. We could have gone faster while saving fuel.
keep a proper distance to the cars in front of you. I know this is often hard when the other guy is going so SLOWLY, but it really helps saving and safety.
If a car in front of you has to slow down in order to turn and you have no distance, you will have to slow down as well. Re-accelerating uses your expensive gas.
EDIT: I forgot this one but it's important:
Try to make little use of your air conditioning. AC needs an incredible lot of gas. Heating is fine as it uses waste heat the engine produces anyway.
Set your AC a couple degrees higher and you will save.
Open the doors only for short times when AC is active or your cool air will escape and you need more gas to make new cool air.
Tips for manual shifters:
Always go in highest gear possible. This really saves a lot and your engine loves the low RPMs.
accelerate fast while switching gears up fast When accelerating, push the gas pedal down 3/4 of the way. Most engines are most efficient around this point, by far. (only accelerate if it makes sense to accelerate of course)
go to neutral and roll when it makes sense. You wouldn't believe how far your car will roll, while the engine uses next to nothing in idle. Works especially well when going down a light hill, or on the short way to the next red traffic light.
If you want to make saving as much as you can a hobby and there's no traffic you would bother, you can alternate between accelerating in highest gear and rolling. This way when you use your engine it's in its most efficient state possible. The difference this makes is bigger than you might think.
Many engines can be as efficient as 25% (diesel 35%) but in normal driving will be around 10% or less :sick2:. That's where the difference comes from.
By the way, when going at a constant speed most cars use the least gas when you drive 35-45 MPH in highest gear.
Going slower the friction in the drive train burns your gas (especially when you need lower gears), going faster the air resistance increases.
Going faster than 80 the air resistance increases A LOT.