Tell me if this is a good investment (CNG tanks)

Let's say I get a new vehicle in the somewhat-near future:

A CNG tank would cost me $11,000.

I would get a 50% tax credit the next tax season.

I currently spend about $80/week on gas, and natural gas is consistently between 40-50% of the cost of gasoline. Also, there is a gas station with CNG pumps very close to my workplace, so it wouldn't be inconvenient to fill up.

Let's conservatively say that NG is 50% of gasoline, so I will save $40/week.

I would break even after 2 years, 9 months (($5500 tax credit + ($40/wk X 137.5 weeks) = 2 yrs, 9 mos)).

After that, I would begin saving $2080/yr, or an 18.9% return on my initial investment of $11,000.

What am I missing?? - Obviously, if gas prices go down, my savings per week will go down. Is there anything else I'm not thinking about?