First-year choice
You must elect the standard mileage rate in the first year a vehicle is placed in service in the business. Failing to elect (or choosing actual expenses with depreciation) locks you out of the standard rate for that vehicle's life.
Switching from standard to actual
You can switch from standard to actual in a later year, but you must use straight-line depreciation for the remainder of the vehicle's life — no more accelerated MACRS or Section 179.
Switching from actual to standard
You generally cannot switch from actual to standard once you've claimed any accelerated depreciation (MACRS, Section 179, bonus). The switch is locked.
Year-by-year decision
When you have flexibility (you started with standard), run both methods each year. Standard mileage usually wins for high-mileage, low-operating-cost vehicles. Actual usually wins for high-cost or heavy-use vehicles where depreciation, gas, insurance, and repairs add up.
Lease vs purchase
Leased vehicles can use either method, but if you choose standard mileage, you must use it for the entire lease period. Switching mid-lease is not allowed for leased vehicles.
Common mistakes that disallow the deduction
The recurring ways this deduction gets disallowed in examination cluster in four categories: (1) personal-use expenses bundled with business (the deduction is disallowed entirely or apportioned downward); (2) inadequate substantiation (no receipt, no invoice, no business-purpose note); (3) the wrong line on Schedule C (not fatal, but it weakens audit defense); and (4) double-counting with another line (for example, deducting an expense on Schedule C and also on Form 8829, or as a personal itemized deduction on Schedule A). The fix in every case is contemporaneous bookkeeping and a clean chart of accounts.
Where this fits in the larger Schedule C picture
Schedule C has more than two dozen named expense lines plus an "Other expenses" catch-all. For most small businesses, four or five lines drive the bulk of the deduction total — vehicle, home office, depreciation, contract labor or wages, and supplies — and the remaining lines individually contribute small amounts that nevertheless add up. Treating each named line as a recurring decision rather than an afterthought, and revisiting the categories each January, often surfaces $2,000–$5,000 in additional legitimate deductions that a less disciplined process would have missed entirely.