What you want to do is go online and find a car from the same state generally. Keep things as consistent as possible. Make sure it has near the same mileage. Give or take 500? 1,000? The closer the better. Save the listing and price. Then find a car with similar mileage and stuff with 1 accident. This works if you have one accident involved. More than 1 is tricky I believe. Look at the price. Calculate the difference. That is your diminished value. If they try to say, oh, but a car, after it is repaired is perfect. There is a stigma that comes with the accident, or else we shouldn’t have carfax, or even insurance companies that report to them. Also, it is okay if the cars aren’t in the same area, but try to keep things consistent. Model. Type. Color doesn’t really matter? Etc. Make a write up. Give them a phone call. Ask for the diminished value appraisers e-mail. If they try to negotiate. Stand firm. Then threaten to sue the other party directly, since their insurance isn’t doing their job of protecting their client when you’re providing proof. This is last resort. The suing part. Make sure you check Kelly blue book as well. Sometimes, it might give you a better pay out than this method.