Technical stuff about car deductions. If math makes your eyes glaze over, you might want to skip this:
The best thing to do is calculate it both ways.
It *is* possible to switch methods under some circumstances. (If you switch to Actual Expenses, you have to recompute your basis in the car by adding up the depreciation component of all the Standard mileage deductions you have taken and subtract the total from what you paid for the "business part" of the car. You can then depreciate the rest of it. So it's a bit tricky, but it can be done.) (Switching from Actual to Standard is only allowed if you took straight-line depreciation from day one. Keep in mind that returns can be amended for three years.)
Usually if the car is very old and gets poor mileage you will do better with Actual expenses because repairs will be higher on an older car and the gas is more. If you don't drive the car a lot, a large percentage of those repairs may become deductible. (Example: the car was only driven 1000 miles, 250 were for shopping, and you had a $2000 repair bill. $500 of that bill is deductible under Actual Expenses, but you would only have $140 for standard mileage deduction) On the other hand, if a relatively new car is used just a little for shopping, the mileage deduction may be the better deal.
New cars have high depreciation, but this is capped each year, so the results will be different if the car is very expensive versus a $15,000 Toyota sedan.
It's always best to explore whatever options are available and decide what is best, not just for the current year, but for future years.
9 times out of 10 mileage may be better, but if you're the 10th person, taking mileage could cost you an opportunity to reduce taxes legally.
And if you lease your car, you must take Actual Expenses. You're not allowed to take the mileage deduction because it includes depreciation and you can't depreciate something you don't own. However if it's a lease purchase it may be treated as if it were owned.
And if you borrow a car to mystery shop, all you can take is the actual gas and oil that you paid out on it. You can't take standard mileage at all on a borrowed car, even if you pay the insurance and repairs. You didn't pay for the car, so you can't depreciate it, and depreciation is part of the standard mileage rate.
However if someone *gives* you a car, your basis in the car is equal to the donor's basis -- so you can take standard mileage on it because you own it and it had value when they gave it to you.
If you rent a car, you can deduct actual expenses based on the percentage of business use to personal use for the rental fee and the gas you put in it.
One size does not fit all, especially when it comes to vehicle deductions. But it's true that for most people the standard mileage deduction is best, and minimizes the record-keeping you have to do.
Time to build a bigger bridge.