I'm a sole proprietor/self employed in the oil and gas industry. I use my personal vehicle for work. The actual mileage driven is very small, but I spend 8-12 hours idling in my vehicle when I do go out to a job site. How do I determine what amount of vehicle expenses I can legally deduct? Basing everything off of work miles vs personal miles seems like I'd be losing a lot of deductions power especially in terms of gas.

You're right that the standard mileage method would undercount your actual business use in this situation. The good news is you can use the actual expense method instead, which lets you deduct a percentage of all vehicle costs, including fuel burned while idling, based on the percentage of time the vehicle is used for business.

Here's how it works: instead of tracking miles, you'd calculate what percentage of your total vehicle use is business-related. Since your work involves long idle periods at job sites, time is actually a more accurate basis than miles for measuring business use. The IRS requires you to substantiate the business purpose, date, and amount of each expense, but it doesn't mandate that mileage alone determines your deductible percentage.

In practice, you'd keep a log of your work days, how many hours you used the vehicle for business on each trip, and your total hours of vehicle use overall. If, say, 40% of your total vehicle time is business use, you can deduct 40% of your gas, insurance, registration, depreciation, and repairs. Given that you're burning significant fuel while stationary at job sites, this approach captures that cost in a way mileage never would.

One important note: once you use the actual expense method for a vehicle, you generally cannot switch to the standard mileage rate for that vehicle in a future year. So make sure you're confident this method makes sense long-term before committing to it.