So to start, if you deduct the standard mileage rate, you must subtract non-taxable reimbursement at the end. (You do not need to subtract taxable reimbursement.) If you drove 10,000 miles, that's $5400 deduction, then subtract the $790 leaves you with $4610. That goes on form 2106 along with any other un-reimbursed business expenses.
But then, that deduction is subject to the 2% rule. It is reduced by 2% of your income. If your adjusted gross income is $60,000, then 2% of that is $1200, so your deduction is reduced to $3410.
Then, that deduction goes on schedule A, and you only benefit from schedule A deductions if they total more than the standard deduction. Assuming you are single that's $6300 ($12,600 for married filing jointly.) Your schedule A deductions include state income tax or sales tax, charity contributions, property tax and mortgage interest.
Let's say you are single and rent. Your charity contributions are $1000 and your state tax is $3000. Without the work deduction, you will get the standard deduction of $6300 since it is more than your itemized deductions. With the work expense, your itemized deductions are now $7410. But that's only $1100 more than the standard deduction, so even though you start by listing $5400 of mileage, you end up increasing your tax deduction by $1100. Which will save you $150-$300 depending on your tax bracket.
And, if you have not yet entered your other itemized deductions like mortgage interest or charity contributions, they will improve your refund once you get to them.
The actual numbers will vary but the principle is the same. Since turbotax online hides the forms until after you pay, the only way to see your actual numbers is to go to the File tab, pay the fee with a credit card, then print a draft copy of your tax return without actually transmitting it. Then you can make changes or continue to work on it, and only go back to file when you are ready.
