I'm not an accountant, but heres my two cents.
If you use mileage, you get .36 per mile. That's it. You should have a log or some other way of accounting for mileage. Gas, oil, vehicle insurance, car washes, etc are "rolled" into the .36 cents.
If the vehicle is 100% business, not need to log mileage.
If you have a receipt, you must have went to the store and back. Go to map-quest and get the miles if you didn't log them. I log all my receipts, and then I have all the mileage for those trips.
If you have a mowing route, the same every week, you may be able to just multiply the # of weeks by the mileage.
Either way, you must be able to "substantiate" the miles. A mileage log will do it.
The other way
If you depreciate, you may do it all this year, with the new tax laws. Up to 100K I believe. Then you take all other expenses, gas, oil, insurance, washes, vehicle add-ons, maintenance, and deduct them. Some are depreciated if they are an improvement (i.e. a dump bed), others are expensed (wiper blades, cup holders, car detailing, washes, most things less than ~$200).
Whichever method you choose, you have to stick with it over the lifespan of that vehicle. If you have two vehicles, you can use two methods, one for each.
A used vehicle, one you already own, or a cheap vehicle probably best to use mileage. Lots of miles on a new truck may fall into this category.
A new F350 4x4 diesel plow-truck, it may be best to use depreciation.
Ease of capturing expenses may determine which method you use too.