Currently, there is no option to pay your employees by mile per hour. I have a workaround you can consider.
You can consider adding them as an hourly paid employee. You can treat the hourly pay the same as a pay per mile.
You can also consider adding another pay type for your employee. This way, you can keep track of their pay per mile.
Here's how you can add a pay type:
You may also refer to this article for more information: Supported Pay Types and Deductions Explained.
Please let me know if you have any other concerns.