How is a car's diminished value determined after a car accident?

Answer

Generally by the "book" value. Take into consideration and make mention of any excessive work or upgrades you made to the vehicle and tell the adjuster asap. MUST HAVE receipts to prove this!

If you're speaking of inherent diminished value, then the only way the amount of value the car lost after being in an accident and repaired is to have an expert determine the loss in market value utilizing accepted appraising practices.

Answer:

Diminished value is what the vehicle has lost in value according to reliable market analysis conducted by a third party NOT the insurance co.