Don't look now folks, but it's Memorial Day weekend which can only mean three things – barbecues, an extended weekend, and the inching higher of gas prices.

At roughly $3.67 per gallon as of last week, according to the Energy Information Administration, gas prices have edged slightly down from their highs, but they're also only fractionally lower from the year-ago period and serve as a reminder that the scarcity of our natural resources is only going to place more importance on fuel economy for cars in the future.

The rise of eco-friendly vehicles
Over the last couple of years we've really witnessed a push by regulators, as well as automakers, toward vehicles with higher fuel efficiency, including hybrid vehicles which run on an electric motor up to a certain point and then switch over to a gas-powered engine when the electric capacity has run out. We're also seeing automakers pushing out all-electric vehicles in an effort to appeal to socially conscious consumers, as well as demonstrate that the technology is not only viable, but can save consumers money over the long run.

This is both a necessary move for automakers, as the government has set clear regulatory guidelines on where it'd like the average mile-per-gallon value to be in a decade, and a smart move because consumers judge a brands' image based on how wisely it uses global resources.

As my Foolish colleague and alternative energy guru, Travis Hoium, pointed out this past week, Tesla Motors (TSLA -1.11%), despite the hefty cost of its all-electric Model S which can occasionally top $100,000 when all options are included, could save a consumer that drives 15,000 miles annually $261 per month compared to a car that gets roughly 20 mpg.


Source: Tesla Motors

Green cars both pull at the heartstrings of those who want to do better for the planet, and they offer cost-conscious consumers the genuine potential to save money over the long term. But, can these vehicles and the companies that manufacture them also serve as viable investments?

The top three green cars for 2014
Just last month the auto experts at Kelley Blue Book released their annual findings on the top green cars for 2014. As you might have expected, the Tesla Model S made the list, but it only came in fifth overall. Today, we're going to take a closer look at the top three green models according to KBB and determine if there's genuine growth to be had for these pioneer vehicles, or if they merely have flash in the pan potential. 

No. 3: Toyota Prius
There is perhaps no better-known hybrid on the market than the Prius from Toyota (TM 0.60%), which has been in production since 1997 and is currently deep into its third-generation design with a fourth-generation Prius due out in 2015. Not only has Toyota managed to improve the fuel efficiency of the Prius itself, but it's also greatly expanded the line to include the less costly Prius C, the lengthier Prius V, and the Prius Plug-in which can travel 11 miles solely on electric power.


Source: Toyota

But will it continue to sell is the $64,000 question?

Prius sales rocketed higher in 2012, blazing to 236,659 units in the U.S., a hair more than 100,000 more than it did in the year prior. Sales in 2013, though, tapered off by about 1%. Consider this, though: The Prius hasn't had a major redesign since 2009 and Toyota's on the precipice of introducing a new model next year that could potentially be lighter and even more fuel efficient than its predecessors, so the minor drop-off in sales isn't all that surprising. Not to mention, if Toyota chooses to go the route of using the slightly more expensive lithium-ion batteries, then the car could be lighter and consumers may find it more spacious.

With few hybrids tipping the scales at the 50 mpg mark while also remaining affordable, the Prius looks as if it'll remain a mainstay of success for Toyota. That also means that investors should keep Toyota on their radars as the new fourth-generation Prius is introduced.

No. 2: Nissan Leaf
No big shock here, as the Leaf from Nissan (NSANY 1.14%) took the top spot in terms of highway fuel efficiency in a recent Consumer Reports study with a whopping 118 miles per gallon equivalent, or mpge. KBB notes that the reasoning behind its No. 2 green ranking is that the Leaf can deliver 84 miles of electric range in one charge, and is priced at less than $30,000 prior to the whopping $7,500 federal tax credit. That affordability, combined with a "straightforward interior" makes the Leaf a practical consumer car in KBB's view.  


Source: Nissan

But, can the Leaf keep outselling other electric vehicles?

Given how quickly Tesla has been able to ramp up its Model S sales and surpass expectations in multiple quarters, the Leaf's domination as the best-selling electric vehicle is likely to come to an end. But that doesn't mean the Leaf won't have a niche audience to attract for at least the next couple of years. Although the Model S may be superior to the Leaf in electric range and amenities, it also starts at more than double the price of the Leaf, pricing a number of would-be consumers out of the market. The Leaf can continue to serve as an EV with a practical entry level price point, especially for consumers who live near where they work.

Things could heat up for Nissan as Tesla looks to introduce the Model E in 2017, which will be priced somewhat comparably with the Leaf. In the meantime, though, Nissan's efforts to garner EV market share in the U.S. appear to be moving along nicely.

No. 1: BMW i3
However, no company dazzled the environmental enthusiasts at Kelley Blue Book more than the highly anticipated compact from BMW (BAMXF -3.53%), the i3. As KBB notes, the cars' impressive 170-horsepower, all-electric motor gives it some decent get-up-and-go, and you'll find a number of recycled materials within the interior. According to BMW, the i3 will get an all-electric range of between 70 and 110 mpge.  


Source: BMW

But will it sell and help BMW compete against the likes of Tesla?

Early sales figures for the i3 have been encouraging. In October, BMW CFO Friedrich Eichiner noted that there were some 11,000 orders for the i3 in the U.S., which was nicely ahead of the 10,000 it had hoped to sell in all of 2014. This better than expected demand is one reason BMW has considered boosting production of the i3 to meet growing U.S. demand for EVs.

However, the i3 still won't be a slam-dunk at a $41,350 price tag. As Stefan Bratzel, director of the Center of Automotive Management at the University of Applied Sciences, noted in an interview with Bloomberg last month, "BMW invested a lot of money [on EVs and using carbon fiber]. It was a bold move, but it also bears some risk as production is complex." The use of carbon fiber will certainly help with weight and should make the cars look sleeker, which is consistent with BMW's modeling. But, as Bratzel points out, the company will have to remain focused so no bottlenecks in production occur in order for the i3 to be successful in the U.S. For now I remain cautiously optimistic that the i3 will be a hit, but like the Leaf, it could be facing looming competition from Tesla.