those parts you put on the vehicle would add to its cost basis or be repairs. so if you legitimately use the vehicle in your business, ie take a tax deduction for using the vehicle in your business, then there would be a tax deduction based on business miles to total miles. another caveat is who owns the vehicle you or the business. it won't matter if you are a single-member LLC or sole proprietorship. if your business is a partnership and it owns it, the deduction would be at the partnership level while if you own it then it could be an additional schedule E deduction. if owned by a corporation it would be a corporate deduction but if you own it then it would likely be a non-deductible employee expense. the Corp could set up an accountable expense reimbursement plan and reimburse you per business mile base on IRS vehicle mileage allowance.
