Are you using TurboTax? Because they provide some great help articles and resources. Even if you are not using that program to do your taxes, that is a great place to Learn about what matters to you.

Go here:

https://turbotax.intuit.com/tax-tips/jobs-and-career/federal-tax-laws-on-mileage-reimbursement/L6b5I...

I assume you are not preparing your Clients' tax returns, which is this ProConnect Software community you are asking. We can't help really, unless you are using ProConnect.