Feeling the pinch of pricey gas these days? For less than $100, I've got a couple of relatively easy car modifications you can make that will earn you a couple extra miles to the gallon or more, depending on the vehicle.

The first and probably the easiest thing you can do is replace your air intake filter. If you've seen those AutoZone (NYSE:AZO) ads suggesting that you can improve your mileage just by replacing your old and dirty stock air filter with a new one, they're telling the truth. But rather than simply replacing your old filter with an exact duplicate that you'll have to replace again in a year, I suggest that you spend the extra $20 or $30 to upgrade to a reusable, high-flow air filter.

A high-flow air filter is a direct replacement for your old filter and requires no other modification. It allows for increased air flow, additional horsepower gains, and even better gas mileage. As an added benefit, it is also reusable -- when the filter gets dirty, you just pull it out of the airbox, clean it (filter manufacturers offer companion cleaning solutions), and stick it back in. K&N Engineering makes what is probably the most popular brand of air filter, though I can also recommend Green Filter, which I use in my own car.

You can pick up either brand of filter for most applications for about $50 on eBay (NASDAQ:EBAY) Motors.

A second relatively easy -- but slightly more labor-intensive -- thing you can do to improve gas mileage is to purchase a set of grounding wires. Basically, grounding wires replace your car's grounding system and net horsepower and gas mileage gains by reducing electrical resistance. You'll have to get under the hood and figure out the connections, but it is cheap and effective -- more so on older cars than brand new ones -- and it can't hurt the car.

If you're willing to pay up, you can purchase a set of Sun Automotive's Hyper Ground Cables or HKS's Circle Earth System for about $125, or you can pick up a set of Lineage Grounding Wires for under $100. Alternatively, you can pick up a set of generic cables on eBay Motors for about $20 to $30 without giving up much or even make your own kit with parts you can pick up at your local Radio Shack or Home Depot (NYSE:HD) for about $20.

Even putting aside the benefits of having a healthier, better-breathing, more powerful, and smoother-running vehicle, the savings from gaining an extra 20 to 30 miles per tank add up over time. And if you're interested in investing in companies that benefit from people making these kinds of modifications, check out my two-part series from earlier this week -- Investing in Car Culture and Where the Rubber Meets the (Virtual) Road -- in which I take a look at eBay, Electronic Arts (NASDAQ:ERTS), Take-Two Interactive (NASDAQ:TTWO), THQ (NASDAQ:THQI), and Goodyear Tire & Rubber (NYSE:GT).

Electronic Arts and eBay are Motley Fool Stock Advisor recommendations.

Fool contributor Jeff Hwang owns shares of both eBay and Electronic Arts.