(To help envision the potential savings of each of these tips I’ll do a little math using some baseline figures. In order to see how much you could save, lets pretend your car gets 20 MPG, you drive about 12,000 miles a year, and a gallon of regular unleaded gasoline costs $3.29, which happens to be the national average as I’m writing this.)

1. Get Pumped Up

Most cars driving on the road today do so on under-inflated tires. You can find out what the proper pressure for your vehicle’s tires is by checking the driver’s side door jamb for a sticker with the information or looking in your owner’s manual. Tire pressure gauges are very inexpensive and easy to use. Most gas stations have air pumps or you can buy your own that will run off your car’s power socket. Checking your tire pressure every two weeks and keeping your tires properly inflated could help you get up to 3% better gas mileage. While that may not seem like much of a difference (from 20 to 20.6 MPG), over the course of a year it adds up to over $50.

2. De-Junk Your Trunk (And Lose the Rack!)

The heavier your vehicle is the more fuel it will need to get from point A to point B. This is one of the reasons big SUVs and trucks use so much of it. One easy way to lighten your load and increase your fuel efficiency is to keep the inside of your car cleaned out. An extra 100 pounds can decrease your mileage by up to 2%. The smaller and lighter your car is the more you will benefit from this. However, not all cargo is necessarily carried inside your vehicle. Large, bulky roof racks can also greatly impact fuel mileage. This is due to both their weight and their poor aerodynamics. Removing a large accessory rack from your car or SUV can provide a 5% or greater increase in MPGs. Taken together, these result in a savings of $130 annually using our figures.

3. Slow It Down

When driving down the interstate peer pressure is a powerful thing. When everyone else is doing 75 MPH it feels foolish to not do the same. However, slowing down just a little bit on the highway can provide a huge savings in fuel economy. Under about 45 MPH the primary resistance a car encounters is rolling resistance between the tires and the road. Above 45, air resistance becomes the biggest factor. Air resistance increases exponentially with speed, so the faster you go, the more your gas mileage drops. The math on this one is difficult to estimate, as it is hard to tell what percentage of your driving is done on the highway, at what speeds, and how aerodynamic your vehicle is. According to the U.S. Department of Energy it’s safe to say that each 5 MPH you drive over 60 MPH is like paying an additional $0.20 per gallon. Let’s say that you switch from driving about 3,000 miles annually at 75 MPH to doing the same at 70 MPH. That’s at least a $30 savings. Plus it’s safer too …

4. Breathe Deeply

Ok, this one may require getting your hands a little bit dirty. But just a little bit, I promise! In order for your engine to operate as efficiently as possible it needs lots of fresh air. Your car’s air filter makes sure that air is as clean as possible when it enters your engine. After a while the filter begins to get clogged, which is not necessarily a bad thing because that means it’s doing its job! Once the air filter starts to get dirty your engine can’t get as much air as it needs, and it begins to run less efficiently. Most manufacturers recommend replacing your car’s air filter every 12,000 to 15,000 miles for peak efficiency. This may seem like a lot of air filters, but replacing a dirty air filter can increase your car’s mileage up to 10%! That’s a jump from 20 to 22 MPG and a savings of $180 in a year, which is definitely worth the cost and trouble of replacing a $15-$20 air filter.

5. Don’t Sit Idly

Excessive idling can wreak havoc on fuel mileage. Remember, when you are sitting still idling you are getting exactly zero miles per gallon. You may have heard that starting and stopping your engine repeatedly will produce excess wear. While this can be true, if you have a relatively modern car and take care of normal maintenance like oil changes and tune-ups then you have nothing to worry about. Here again the size of your car can make a big difference. The larger your vehicle’s engine the more fuel it will use while idling. Sometimes idling is unavoidable, but times like sitting in line at the drive-thru or waiting to pick up a friend are perfect opportunities to shut down your engine and get more for your gas money.

Assuming all of these tips apply to your situation, these 5 tips combined could result in a whopping 20% increase in miles per gallon. In our hypothetical scenario that’s a potential yearly savings of around $350! I know there are plenty of other ways to increase your fuel mileage and decrease the amount of money you spend on driving around, so feel free to share some of your favorite tips and tricks in the comments.

And as always, your mileage may vary …

Friday5 is a weekly column that lists five items on a theme drawn from one of the various categories covered by From Bits to Bites & Windshields to Worship.

As we contemplate what car to buy when it is time to replace this one, we are facing many of the same factors as other car-buyers in the US. Gas prices are high and are only predicted to go higher. The economy is heading downwards and that means there is less money to go around in general. I also worry about the continued detrimental impact on the environment of burning so much gasoline and the amounts of hazardous chemicals that go into making the batteries of all these new-fangled hybrids.

Since this will be our first “big” car purchase, we have already begun contemplating some of the options that will be available to us.

Option 1: Used VW Golf (or Jetta) TDI

We really like the small diesel-engined Volkswagens, such as the Golf and the Jetta. We have known some people who have had such cars and they report regularly getting between 40 and 50 miles per gallon. There is a local fueling station that carries soy bio-diesel, which would further help with environmental impact issues. However, these are very popular cars right now and they are quite difficult to find in the used market with low miles and reasonable prices. In addition, VW has yet to release a diesel version of the new Rabbit, which replaced the Golf a couple years ago. And so, while this remains an option, our chances of finding one of these in our price range in good condition with low miles are very slim.

Option 2: 2008 smart fortwo passion coupe

Though the smart fortwo is a rather unorthodox little car, we are both open to creative solutions to our car needs. We have actually gone so far as to visit the smart dealership in Indianapolis to test drive one of these. (You can read about our visit at my wife Becky’s blog.) I’m impressed by the engineering that has gone into making this tiny little 2-seat micro car. It has boatloads of technology such as brake-assist, traction control, stability control, etc. It is even relatively fun to drive!

The fortwo is not without its shortcomings however. I was less than impressed by the sometimes-sluggish gear changes from the “automated” manual transmission. In addition, its EPA mileage ratings aren’t really that impressive (33/40) considering all the sacrifices you make for such a small car. The first owners of the new-to-the-US 2008 fortwo have just begun taking delivery of their new cars, and so I am anxious to see what sort of mileage they experience in the real world. I am also a bit put off by the fact that they only come with a 2 year warranty and I fear that parts for them could be rather expensive. Another potential downside of the smart is that they are currently in very high demand. There is currently a long waiting list and it could potentially be a while before we would be able to get one.

Don’t get me wrong, I really like the smart and all that it stands for. It’s truly all the car that most folks would need for the chores of daily commuting and running around town. I also really like that they are concerned with environmental issues surrounding the entire production process. However I am interested not only in the environmental aspects of getting a more economical car, but also the financial aspects. The smart fortwo, with the options we would want, retails for about $15,000. There are quite a few cars in that same price range that get 75-80% of the gas mileage the smart gets, but have amenities such as cruise control and a back seat.

Option #3: 2008 Nissan Versa Hatchback SL

Since I’m not feeling very confident that we’ll be able to find a Golf or Jetta TDI, and I’m having second thoughts about the smart, I’ve started doing some research on other cars in the same price range. I still want something small and very fuel efficient, but also able to go on weekend road trips when we might want to leave our larger car at home. My research has recently led me to look more seriously at the Nissan Versa hatchback. For about the same money as the fortwo, the Versa SL with a 6-speed manual transmission is rated for 26/31 miles per gallon. Not too shabby for a small wagon with a back seat and a usable trunk. If you opt for the CVT instead of the manual the mileage rating rises to 27/33. Part of the draw of this car is the 6 speed though, so the CVT would have to be really good in order for me to forgo the manual.

I’m drawn to the Versa because it doesn’t come across as your typical, stripped-down econobox. In other markets, where it has been sold for several years as the “Tiida,” it is actually marketed as an entry-level luxury car with options such as navigation and leather seats! With all this in mind, the nagging questions that I keep coming back to are long-term cost of ownership and practicality.

I imagine that whichever car we choose it will be driven approximately 20,000 miles a year or less. After doing a little research and some math, the Versa looks like it would probably cost about 20% more annually for fuel. At a pessimistic $4 a gallon that translates to about $500 a year. At $3.50 a gallon it becomes a $400 difference annually. Real-life numbers would inevitably more complex than this little exercise however. Getting a slightly larger car like the Versa would mean that we could use it for longer trips, thereby saving money by not taking our larger car which only gets about 28mpg on the highway. It’s not a huge difference, but every bit counts. Once you start to account for driving a somewhat more efficient vehicle more often rather than a much more efficient vehicle less often, it is difficult to discern what vehicle would make for the best overall “fleet economy” for us.

So how wise of a purchase would a smart be, all things considered? For now the answer seems unclear. There are many factors that will play into our decision that are still a bit murky. How high will gas prices go? What kind of mileage will drivers really experience in the new smart? How difficult will it be to get one? Would we be comfortable taking it on day trips to Indianapolis or Dayton on the busy interstate? Only time will tell. I’d be interested in hearing your thoughts in the comments section. Stay tuned over the next few months, because I have a feeling this could be a bumpy ride.