Now here's the good news:

If your fuel expeditures is big enough so that your ROI is 11 months (as assumed in your computations), then you would save an additional 30k on your second year

This would mean that you would be "earning" 30k a year after your 1st year. This is provided that you maintain your fuel expenses and you fuel requirements within your assumed computations, and no costly maintenance work on the LPG system takes place... If you consume even more fuel, then the "savings" should be even bigger.

For those who dont consume as much fuel, or have found other ways to "save" fuel (such as public transport, or find work/school at a nearer location from their home), then the numbers would be different.