Center of the front wheel to the center of rear wheel distance.

 

On a SuperCrew you will quickly notice the EXTRA 12 inches of bed in FRONT of the rear wheel well.
Or put another way, the bed is an extra foot long and that extra foot is between the wheels, so it's visibly different between the back of the cab and wheel well.

Note red line on each truck

 

Look at one of each side by side and you'll never have to ask again. It may only be a handful of inches, but the difference is quite obvious.

 

And the white sticker in the door shows the WB; if you trust the door sticker then no need to measure.

 

The tape measure is a highly under-utilized tool.