if your wife is not using the car for business purposes, then you converted it to personal use. you are deemed to have sold the car at FMV on the date.  the total FMV is divided split between the business FMV based on business mileage to total mileage as is the business portion of the cost. the rest of the FMV and cost is the personal portion which you can ignore.

the standard mileage rate includes a depreciation factor each year. IRS pub 463

2020 $0.27
2019 0.26
2017–2018 0.25
2015–2016 0.24
2014 0.22
2012–2013 0.23
2011 0.22
2010 0.23
2008–2009 0.21
2007 0.19
2005–2006 0.17
2003–2004 0.16
2001–2002 0.15
2000 0.14

so you multiply each year's business mileage by the rate above. the sum of these becomes the total depreciation taken

the business portion of your cost reduced by the business depreciation is your net tax basis in the vehicle (not less than $0)

FMV of the business portion of the vehicle less the net tax basis is your taxable gain or loss. 

Thank you.  Could you clarify these?

What I paid for the car when I bought it does not enter into the calculations ?  Only the FMV when I stopped using it?

This is a 2016 vehicle.   I didn't start my sole proprietorship until late 2017.  I claimed business miles in 2017 (207), in 2018 (1058) , in 2019 (2069) and a portion in 2020 (1056) which totals 4,390 business miles.  In your reply --   "the total FMV is divided split between the business FMV based on business mileage to total mileage as is the business portion of the cost." -- does this mean total business miles 4,390 to total miles on the car (since 2016) when I stopped using it even though the first almost two years I wasn't using it for business?  

Thx

Your FMV is probably the same as the sales price. Compare business miles to total miles for the years the car was used in business.

That's the problem. I did not sell the car per my original message.  I bought a new vehicle and stopped using the old one for business.  My wife now drives the old vehicle.   How can I find the FMV of a vehicle as of last March (2020) that I did not sell?  If I enter data about the vehicle in Kelley Blue Book or Edmunds I can find the CURRENT FMV estimate - but you can't look up FMV for a previous date.   Any suggestion?  Just use the current one? 

Thx

to Mike 9241

Please see your reply to my original question.  I understand what you were saying but there is no indication where (on what form) to enter the data.

It's not in the interview.  Is it  the car and truck expense worksheet and if so what sectioned lines in the worksheet?  Or is there some other form?  No instructions whatsoever.