Mpg for Life of Car

Nop! If you reset it, that data is gone from the memory. Even the monthly Eco-Diary is only retained for a max of 12 mo, after that the oldest month record is written over with a new record. Since your car is less than 1 year old, you can calculate the current lifetime mpg from the monthly record, if you have forgotten the last record of mpg with ODO. Then you can also calculate the current lifetime mpg. But, that would be too much work for most. The simplest thing is just to start over and enjoy the inflated mpg on the display, as you may know, that number does not reflect the true mpg of the gasoline engine alone operation (i.e. HV mode).

 

On the other hand, you have 3 trip meters to look at. So, if you've only reset one of them you still have two others to look at. Again, the other two (trip 1 and trip 2) will roll back to 0 miles after each 10k miles (if memory serves), BUT, will keep the mpg calculation from the ALL previous miles counts as long as they are NOT ALSO manually reset.
Depending on your MPG OCD level - and or - your desire to understand ALL the gauges the Prime has available, anyone can keep a spreadsheet of whatever interests them about the car, or take pictures of the dash gauges at whatever interval they choose (weekly, monthly, yearly, etc.) to look back at if and when another reset of a meter unintentionally occurs.

edit Keep in mind that several of the meters available on both the Speedo and the meters and graphs available on the Multi Information Display (MID) to the right of the Speedo, are also for (single driving sessions) meaning from the moment the Prime is powered on into READY mode, until the car is turned off.

 

Start a spreadsheet, using the odometer and fill up data. And don't trust the car to display accurate mpg; it readily apparent someone at Toyota got it to display optimistic.