I think the default is the government reimbursement rate. This year it's $0.58/mi. When I worked at a pizza place in high school drivers were reimbursed at this rate. That's a similar market because pizza delivery drivers drive their personal vehicles and it doesn't matter if you drive a Prius or a Hummer.

https://www.irs.gov/newsroom/irs-issues-standard-mileage-rat...