Ok - so you trade in your old car - correct?
So I want to be clear - if you report the sale of the old car and purchase of a new car SEPARATELY
you want the gain realized on the old car to be deferred.
If you have two separate transactions - your purchase price is $110,000
Assuming placed in service on March 30, 2015 - first quarter
Depreciation table to be used - see page 72
Table A-2. 3-, 5-, 7-, 10-, 15-, and 20-Year Property Mid-Quarter Convention Placed in Service in First Quarter
That will be 5 year property - so first year depreciation is 35% or $38,500
However because of section 280F limitation - see page 62
depreciation for the first year the car is placed in service is limited by $11,160 if you choose section 179 deduction or $3,160 without section 179.
If you have a short tax year, you must reduce the maximum deduction amount by multiplying the maximum amount by a fraction. The numerator of the fraction is the number of months and partial months in the short tax year and the denominator is 12.
So these limits are reduced as 10/12
The actual limit woudl be
$11,160 * 10/12 = $9300 if you choose section 179 deduction.
If you choose bonus depreciation - section 280F limit woudl not apply and you may deduct 50% of the cost or $55,000.
On your old car - you will recognize neither the gain nor loss.
50,000(selling price) MINUS $50,000 (adjusted basis = 70k purchase price minus accumulated depreciation 20k) - so the gain is zero.