Living within your means and delaying gratification is a good way to build wealth, and I liked your specific example of using a car until it dies rather than a bells and whistles car that depreciates.
However, I think you are mistaken where you quoted $2.5 million at 6% return.
I ran your numbers in excel with the same initial inputs you entered ($7,296 for 4 years). At the double digit 12% interest per annum compounded for 47 years you would end up with $5.1 million as you calculated.
However at a more realistic 6% rate of return you would not end up with $2.5 million you quoted. You would only end up with $414,000.
At 3% interest you would end up with a mere $112,000.
So you would still end up quite wealthier, but not quite a millionaire.
Thanks for the writeup.