Tesla wants (and needs) every customer it can get. However, the most important customers they can get are the ones who are buying the car for the cause, not for the Fuel Savings.
Who spreads more good marketing words? People who drive EVs to save money on their fuel cost, or people who drive EVs for the betterment of the world?
Dropping gas prices will impact sales, yes. But, initial reservations will be high enough that it probably won't matter for a few years. It will take a while for Tesla to be able to put cars on lots that can be driven away that same day.
In the US, the Model S was the only BEV or PHEV that had been on the market through all of 2014 that had an increase in sales in 2015 over 2014. For other cars, the cost of gas is affecting sales, but not Tesla.
The potential pool of people for most EVs are:
1) eco buyers - out to save the environment
2) incentive buyers - someone seeking out government incentives to buy, this is especially a factor in places that allow EVs to drive in the HOV lane with one person
3) people sensitive to energy costs - this is more true of people seeking cheap cars, the average Tesla buyer today could keep a Hummer fueled without a serious dent in their household budget if they wanted to
The mainstream car market is still full of people looking at ICEs. Most hybrids have major tradeoffs for a bit better fuel mileage, but the Prius is the only hybrid that sells anywhere close to pure ICE cars. Additionally hybrids and BEVs usually have poorer performance than ICE. The Prius takes around 10s 0-60, and that's not unusual. There is a meme out there that if you are going to be eco friendly, you have to give up things and live a degraded lifestyle. This is perpetuated by a lot of the eco people themselves (I hear it all the time around Portland which is a super eco conscious city). The car makers just help fuel that meme by providing cars that are a compromise and not at good as ICE in a lot of ways.
So EVs stay in the ghetto of the three groups outlined above.
However Tesla competes head to head with ICE. You do have some trade offs, road trips aren't quite as easy because fueling stops take longer and there are more of them because of less range. However, Tesla outperforms the competition in every other area: acceleration, torque, cargo capacity, noise levels, etc. There are people here in the forum who are eco buyers, but the bulk of the public isn't. Making something that is good for the environment and better than the non-environmental alternative is the way to go if your goal is to get the world to convert. Right now Teslas can be dismissed by many because of the price, but that excuse will evaporate with the Model 3 and I think it will sell well regardless of the price of oil. Other EVs might go by the wayside because not enough people buy them, but Tesla is pretty much immune to the price of oil because they make something better than an ICE.
