I rec’d antique car as a gift from mother and then sold it. She inherited it. How do I correctly claim this income & on what form? Is there a guide to help w this calc?

You are going to take your mother's adjusted basis as your basis (i.e., your basis will be the same as her basis).

Her basis is likely the fair market value on the date of death of the person from whom she inherited the car. You will most likely need to do some investigation as to that valuation.

There are a few things you would need to know to determine YOUR adjusted basis.

1. What was the value of the car when your mom received it?  This may have been documented somewhere in legal papers when inherited, but if not, then you will have to do a bit of research on it.  You can use a site like Classic Car Value to find the 5 year pricing history on a car.  If it was longer ago than that, that either you received it or she inherited it, then you will need to do a bit more digging.  

2. What was the value of the car when she gave it to you? You can use the same method above to find this information.  

Your adjusted basis will depend on if you sold it for a gain or a loss.  If you sold it for a gain, then you would use your moms basis in the car (the value when she received it plus or minus any work she may have done to it)

If you sold it for a loss, then you will use the fair market value on the date that you received it, plus or minus any work you did to it. 

You would have to ask your mom if she paid gift tax on it. 

The easiest thing to do is to get the FMV when you received it, get your moms adjusted basis, ask her if she paid taxes on it and then enter it all into TurboTax to figure out if you have a gain or a loss.