Your question should be what is the proper way to legitimately get the maximum business tax deductions by purchasing a vehicle
to be used for mostly business purposes?
The answer to that is to register the vehicle in the business name, insure it under the business name, but only claim 80% of the expenses related to maintaining the vehicle.
So the answer to your direct question is NO - you can't deduct off all the payments as an expense because of the mixed use.
Anyway, paying off installment notes is paying off the debt to purchase the vehicle. It's depreciation of the vehicle that's applicable to the business use and business % of the operating expenses to maintain the vehicle that are the factors to consider.