Ask yourself: Why did the service manager fess up? Honesty? No. So he could offer you a pittance of a settlement, have you buy off, and then relieve himself of what he must see as a significant liability- you would have accepted the settlement, right?
Also- I would bet $$ that the service managers BOSS or the dealership owner were NOT told about this- and that the service manager is trying to hide what happened on his watch, with his employee, with some free service he can cover. You need to be talking with the owner- not an employee.
The facts:
1. It's a public record that the employee had the car after hours- therefore the dealer is liable for not maintaining reasonable control over a customer's car.
2. Ditto on the public record, the car was "abused" (speeding could be considered by definition "abuse" to the law- 80 MPH is "dangerous" otherwise why have the speed limit?)
3. The ONLY reason the car was taken was BECAUSE they had your keys. They are liable. They hired this bozo and are responsible for his actions. And they know it.
4. You have no way of determining what if any damage or wear was caused during that 60 miles. Inspect all you want, there is no way to measure it. You generously offered to allow for an extended warranty as your guarantee.
Any court would be 100% in your favor.
I would talk with the owner, and generously offer to pay them a reasonable per mile charge for the miles YOU put on the car ($ .20?). Then, they give you a NEW GT as a replacement.
Sounds ridiculous? It won't to a court of law, and their lawyer will tell them so. To them- they will know they can sell your car for near as much as they could have sold the replacement.
But only the dealership OWNER will see this for what it is- a clear case of negligence with some BIG ramifications- the media, the courts, the works.