Since you never used the vehicle 100% for business, that means you took the "per mile" deduction each year it was "in service" at whatever percentage. As you know, the per mile deduction changes each year. Now included in that per-mile deduction is depreciation per mile, which also changes each year. Again, as you know the total depreciation has to be recaptured in the tax year you sell the vehicle. To get the correct amount of depreciation, see the "depreciation per mile" chart at https://www.smbiz.com/sbrl003.html#dsm which covers all tax years back to 2000.
Remember, this is like you're reporting the sale of two vehicles. For the business portion sale you'll use the average business use percentage to determine what percentage of the cost basis is business use. The overall cost basis is not what you paid for it originally. It's the FMV of the vehicle on the date it was placed in service, and you[ll use whatever business use percentage of that as your cost basis for the business side of the sale.
End the end, because it's a given that you did not sell at a profit, you will be taxed on all recaptured depreciation at least up to the sale price allocated to the business side - whichever is higher.
So if you have $2500 of depreciation and you sold the vehicle for $2000, you can expect to be taxed on $2000.
Whereas if you have $2500 of deprecation and you sold for $3000, you can expect to be taxed on $3000.
On average, the vehicle is entirely depreciated in 5 years (or the business use portion of the vehicle is fully depreciated in 5 years). It just depends on the actual business miles driven and has nothing to do with the percentage of business use when it comes to the recapture of depreciation.
