I have a 2013 JK Wrangler. After driving it sometimes the Reset button would not reset the MPG, and it would always reset the timer.
After 4 times to the dealer, with multiple conversations with Jeep Customer Service - the net result is they don't plan to fix it. So -
If you find sometimes the Reset button does not reset the Avg MPG, do the following. Go into the Menu, sroll down to anything EXCEPT the Timer, and then exit out using the North button. This will clear the locked up condition and your Average Mpg will reset as it should.
DO NOT exit out of the Menu screen on Timer - or it'll lock up the Average Mpg Reset function.
Hope this helps. After the forth time to the dealer I asked if we could ride in another new Wrangler, and that vehicle did the same thing.