Seems like an easy enough thing to measure...
From the back of the front seat (fattest part, about 2" below the head rest) to the front of the back seat (same horizontal distance below the head rest).
Normal seating positon - 27"
Seat all the way forward - 34"
Seat all the way back - 24"
Normal seating positon - 29"
Seat all the way forward - 36"
Seat all the way back - 26"
Note* My wife (passenger) likes to have her seat slightly less reclined than mine (driver) so that accounts for the different measurements.
Frank - 2011 Volt #3470 in So. Cal.