The above answers assume you are a disregarded entity (sole prop or single member LLC reporting your business on schedule C). If this is a multimember partnership or S-corporation, and you are using your personally owned vehicle for business, things get a little more complicated.
Assuming you are a sole prop or single member LLC, and the vehicle is used 100% for business, then you can use the standard mileage method or the exact expense method. If you use the standard mileage rate in the first year, then in future years you can change between standard mileage and exact expense. If you use the exact expense method the first year the vehicle is placed in service, you must always use the exact expense method.
All the rules are in chapter 4 here.
https://www.irs.gov/forms-pubs/about-publication-463
Note that your loan interest is a deductible business expense no matter which method you use to deduct the actual vehicle operating expenses.
Some thoughts on depreciation. Depreciation is an allowance for wear and tear. If you use the exact mileage method, you will depreciate the vehicle over 5 years. That means even though you spent $35,000 this year, you only deduct roughly $7000 per year as a business expense. You may qualify for section 179 depreciation, which would allow you to deduct the entire $35,000 in year 1. However, if you stop using the vehicle for business in less than 5 years, you will have to repay some or all of the depreciation you claimed.
The standard mileage rate includes an allowance for depreciation of 27 cents per mile. The trick is, if you use the standard mileage method, you can get the benefit of that depreciation allowance even after the vehicle is fully depreciated. Suppose you keep this truck for 10 years. You can fully depreciate it in year 1 under section 179, or over 5 years under normal deprecation, but for the last 5 years, you get no benefit from depreciation. However, suppose you drive this truck 20,000 miles per year and use the standard mileage method. Over 10 years you will claim $54,000 of depreciation as a business expense even though you only paid $35,000. Obviously, it depends on the mileage you expect to drive and how long you expect to use the vehicle for business. But if you drive the vehicle far enough, the standard mileage method may have advantages in the end, even though you can't depreciate as much up front.
