I drive an old 89 Corolla, and I get 35-42 MPG consistently. On heavy freeway driving, it's on the high end, on a mixed commute it's on the lower end of that range.

Here are the things I've done to get that performance: (not in any particular order)

1. Advance engine timing and use high octane gas. To determine whether or not that gas makes sense, you have to do the math to convert your costs to dollars per mile. Then you know. In that car, with the timing advanced some, it makes a lot of sense. YMMV, particularly on newer cars that have much more advanced engine control.

What you are looking for is the timing point that delivers a fast cruise, but does not ping at moderate load / low RPM driving. On the freeway, the higher speed cruise means less gas / mile, the trade-off being performance. Pickup speed won't be as great as it would be with the factory timing, but overall consumption will be less, and on some cars, significantly less.

2. As mentioned above, lighten the load. One of the reasons I like to drive pre 1992 cars is they tend to be light. In any case, carry as little as possible.

3. Use a manual transmission. In some newer cars I've driven, the automatics are actually quite good, but they are not optimal with a skilled driver.

The manual trannie allows for a few great gas saving cases:

--Low RPM moderate speed driving. Think 30 - 45 in 4th or 5th gear, where you just barely touch the gas. Requires a downshift for any significant acceleration. This becomes habit quickly, with the downshift easily done ahead of time.

--Shift to neutral to roll, capturing kinetic energy. Some topologies don't benefit much from this. The flatter things are, the less of an advantage this is.

--In town, quick burst, roll to stop. I'll do this between stop lights, and in areas where there are many stop signs. Shift into gear, speed up, neutral, roll to near stop. This also quickly becomes habit. On some cars, a start in second helps with this, on others, it needs to be just first, or a quick bump to speed in first, followed by a sustained pulse to reach necessary speed in second.

4. I actually prefer throttle body engines for the low RPM, moderate speed case. Many injection systems have too rough of a fuel input pulse for it to make good sense. The analog nature of throttle body engines allows for more fine variation in fuel delivery, capturing some good economy for that case. As this is half my commute, it's significant.

I've noted newer cars are significantly improved. I recently got near 50 MPG driving a Ford Focus 2010 model from SFO to SAC, and it was impressive! The transmission ratios they selected are excellent, as is the lower RPM moderate speed case. I may well get one of these cars, if I can't get a Volt. A Focus with a manual trannie can probably yield better in a lot of moderate driving scenarios.

A recent rental of a Sentra proved very similar. Clearly automakers are improving on some of these cases, yielding good results for drivers aware of them. Nice!

I only buy new cars once in a blue moon, as the savings I get driving a simple car with good efficiency is insane! For years, I did not rent vehicles that would improve over the one I had in terms of dollars / mile. Recent gas autos are starting to be worth it. Time for a new car maybe.

5. If performance isn't necessary, larger tires can help on some cars, but that depends on a lot of things. Both my gas guzzler Expedition and frugal Corolla benefit from taller tires. Fewer engine RPM / unit distance traveled is the reason. This depends on driver need for performance and topology. If it's largely flat, this can be a nice gain. If it's aggressive, it could be a loss.

Some cars here in the US are now including a 6th gear, or 5th, if it's an auto transmission. Recommended over a gear / tire ratio change as the driver can select the performance / economy driving mode they prefer with little hassle.

6. Do not idle the car. Changing a starter is dead simple in both my vehicles. Doing that every so often is much cheaper than wasting a lot of gas at idle. One exception is in extreme cold, and or the first start of the day to allow lubricants to properly distribute. A good rule of thumb is 1/2 gallon of gas / hour of idle. Larger cars require a gallon, some smaller ones 1/4 gallon. I like to stop the engine for wait periods longer than a minute or maybe two.

7. Engine lube additives help over the life of the vehicle to prevent wear and keep internal engine resistance down. My current vehicle is well over 300K miles, with 50K additive intervals. It performs significantly better than other vehicles from the same year and manufacturer, despite having far more miles.

8. Perform regular maintenance on schedule. Air filter, oil change, trans fluid, brake fluid, etc... These are easy to ignore and they will impact economy significantly, as well as impacting overall engine life. My current vehicle, despite the high miles, runs clean and does not consume much oil, being down perhaps 1/4 to 1/2 quart total on a 5K mile oil change schedule. This is important.

9. Limit driving speed. All cars have sweet spots. They vary too. In general, lower speed overall brings up gas economy. If your vehicle is a newer one with the OBDII diagnostic system, you can get a scanner / USB device to display engine telemetry in real time. The sweet spots are caused by the engine power / torque per fuel delivery curves and the gear ratios in the drive train.

10. Utilize a engine monitor / scanner device. The purpose here is to understand how your vehicle uses gas. In my Expedition, which can use a LOT of gas, using that device brought a commute drive from 12-15MPG up to 17-20MPG. Very significant savings are possible. On that one, I learned there are significant losses in the torque converter, as well as the cruise economy sweet spot being about 63 MPH, not 55. In town for a short distance between stops, that means there is better economy accelerating the vehicle quickly, then allowing it to roll to the next stop, or use light touch on gas to maintain some speed to next stop. That one very seriously improves in town MPG and isn't intuitive. There were many other basic things that device showed me. Paid for itself in a coupla weeks. Recommended.

11. Use your real time MPG display, if your car has one. Just this one displayed metric can improve your MPG by 2-5 MPG by altering driving habits alone.

12. Of course, drive less! Walk to the local store, etc..

13. For heavier vehicles consider look ahead driving, managing speed to hit optimal traffic control device movement interruptions. Say you take off from a stop light, quick start to optimize fuel usage, then are cruising to maintain speed. When you have to stop at the next light, the high cost fuel investment to reach speed again is expensive. Consider looking ahead to the next light. Adjust cruise speed downward, or ideally provide fuel at a level just shy of that required to maintain speed, allowing more time for the light to change, or apply a short burst to get through a light that hasn't changed yet. There are very considerable savings to be had in-town by doing this. The reasoning behind this one all comes down to the fuel investment needed to travel the distance. A long, moderate acceleration at 10MPG can consume more fuel than a short, aggressive acceleration at 7MPG.

Pretend you have no brakes and drive around. That's a great way to get a feel for what I'm writing here. This works on all vehicles, though it works best on heavy ones, and or ones with manual trans, where the losses from torque converters won't detract from the "keep it in motion" savings.

14. Avoid high traffic times. Stop and start and idle conditions waste energy, requiring more fuel.

15. Keep tire pressure correct, or a little high. If you know your driving skill / weather conditions warrant it, lower traction, lower wear tires have better fuel economy than more aggressive tires do. The trade-off is lower traction and lower turning limits on the car overall. If you do bump your tire pressure, you need to check it seasonally, and I personally do it each weekend. Temp variations impact this, and you don't want to be in blow out range on your tire. YMMV. Less tire contact means reduced friction, requiring less fuel.

16. IMHO, simple fuel economy isn't the right metric. Personally, I prefer to consider this question in terms of dollars per mile. Account for what you spend on your car. That's your real cost, and it can very easily mean a car that consumes more gas per mile may be cheaper than one that doesn't, depending on what the rest of your spend is. Insurance, maintenance, supplies, repair, etc... all factor into this equation.

17. Buy a car you can use for a long time. I completely understand that old Corolla, meaning most things that need fixing or replacing I just do quickly over a weekend. The rest I pay for, as my time has value too, and that's how I balance what I do vs what I pay others to do. Timing chain service / replace? Shop. Water pump, or alternator? Me. YMMV, depending on your overall mechanical skill.

18. Route considerations. Shorter isn't always better. You want to keep the vehicle in motion. Some people prefer flats too, but I really don't. On aggressive topologies, I've always gotten better MPG through coasting and pulsing the engine, but that's a preference and skill factor. In any case, some routes will match you and your driving preferences better than others. Avoid stop and start routes.

19. Keep metrics! You improve what you measure. The very simplest metrics are miles per tank full and routes taken. Over time, keeping those and thinking about them will yield some nice savings as one gets more skilled at being opportunistic about how the car is driven.

At any one time, some of these apply. The key to consistent savings, without performing very serious modifications to the car, is to develop the skill on each of them, applying them opportunistically as route, conditions and topology permit.

20. Consider light use of the AC system. Many newer cars have well optimized AC, but there still is a cost. You can pulse these, instead of just leaving them on. Set to max cold, and cool the car, then shut it off. Repeat as needed.

21. Do not drive with windows down. In some weather, this is a trade-off between the increased drag and the AC system cost. Use your OBD diagnostic tool to determine this, or do it empirically by repeating behavior over a few tank fulls of gas.

22. Consider either a hybrid, or a Volt. The two are different, BTW.

At this time, I consider the very high complexity of the Hybrid cars a negative. I prefer long service life and a vehicle I can maintain for the lowest cost of ownership. It may be worth it, if one's driving habits fall in the economy sweet spots these cars have.

To me, a reasonable service life is 10 years 250K miles or so. Anything under that starts to really impact dollars / mile, and that's the real life metric as that represents the real impact of the vehicle on one's liquid income.

In general my experience applying these various techniques has improved on the stated MPG rating by 5 MPG in nearly any car, with some being 10. There are diminishing returns to consider too. A 5 MPG gain in a car that gets 15MPG is a big deal. The same on a car that gets 40, less so.