I self warranty everything nowadays.
Reason being, third party warranties aren't worth the paper they're written on. Have had too many issues with third parties. I used to give 12 months third party on problematic models to wipe my hands of any issues down the line. Trouble is, if the customer has an issue, chances are the warranty company will wiggle there way out of it, saying such and such isn't covered. Then the customer gets back in touch with me and instead of I having to stand over the car for 3 months, it turns into 12 months.
Such was the case recently where I sold a car last January 2016 with 12 months premium warranty. I included it as part of the deal (at MY expense) so didn't charge extra to the customer. In November I got a call from them saying the car was down on power. I informed them to contact warranty company, which they did. They left the car with the warranty approved garage who said it was down to a sticking calliper which wasn't covered. Customer calls me up complaining that they had a 12 month warranty with 2.5 months remaining and what good was it if, it wouldn't cover the problem. I told them drop the car around, which they did. I fixed it at my expense, because they made me feel bad by saying we've bought 2 cars from you in the past, etc etc
Now I give 3 months which is 90 days. I calculate the 90 days exactly using an online calculator. I write the expiry date of the warranty into the invoice so they know at midnight on that date the warranty has expired. I might give a day or two's grace if it's something minor and inexpensive, if the customer is extremely nice when they call up to complain or if they didn't haggle when buying, have bought from me previous, are friends of friends or whatever.
You always get those who will chance their arm and say the car is still under warranty when it could have expired a month or two previous. They must think I don't refer to my invoice book. Then there's those who say the car has hardly been used or only covered x amount of mileage since, or whatever. Usually when I point out in no uncertain terms that it has expired they go away but you always get the few threatening you with court or solicitors or whatever.
Would love to hear how others deal with warranties.