it seems like if you are going to attempt to make a case with the dealer you should have more evidence than a guesstimate of mileage. I would recommend that in the future you take a before photo of the odometer as you leave it so that you know how far the vehicle was driven for sure. A simple iphone photo with GPS and time stamp of THEIR location would be all that's needed to provide conclusive evidence.
Of course there is always the option to take the car elsewhere.
I agree with others.. Other than a sincere apology and a possible minor discount, if there's no actual damage to the car, I can't see the dealer doing much more to rectify the situation. My dealer gives me a loaner car...
and I certainly put more than 20 miles on it while mine is in their shop... It took my dealer 2.5 days to do the battery cable swap... I ended up taking the loaner car on an impromptu day trip that was about 150 miles one direction..so it came back with about 400 extra miles on it..
