just because you have spread your risk out across a fleet of vehicles, and you have made a strategic business decision to balance risk of increased capital expenditures to reduce longer term operating expenses doesn't make the CNG vehicles an investment.

https://www.investopedia.com/terms/i/investment.asp#:~:text=An%20investment%20is%20a%20plan,the%20Next%20Step%20to%20Invest

All of this emphasis is mine. The personal use vehicle never gains appreciation or generates income.

What Is an Investment?

An investment is an asset or item acquired to generate income or gain appreciation. Appreciation is the increase in the value of an asset over time. It requires the outlay of a resource today, like time, effort, and money for a greater payoff in the future, generating a profit.