How do I know how I’m supposed to calculate a deduction for a gas receipt if I use my vehicle for personal and business but mostly for business?

[rjs]

You can't allocate individual receipts between personal and business use. If you are deducting actual vehicle expenses, add up all of your vehicle expenses for the year, then divide the total between personal and business based on your personal and business mileage for the year. For example, if your business mileage is 82% of your total mileage for the year, your deduction for vehicle expenses for business will be 82% of your total vehicle expenses for the year.

You might find it easier to use the standard mileage rate instead of actual expenses. If this is not the first year that you have used this vehicle for business, you will have to check the rules about whether or not you can use the standard mileage rate.