^^ various body builder's guides call for anywhere between 3" and 10" depending on the model of truck and what all they choose to stuff behind the cab.
An example of the larger would be a mate's Isuzu FTR, which has a HUGE plastic intake tube going right up to the top of the cab. It has to be 6-7" diameter, which would necessitate 9-10" between the back of the actual cab sheet metal and the front of the box.
Some of the smaller trucks, like NPR, have "smushed" or ovalised tubing to reduce this gap to as little as 3" between BOC (back of cab) and mounted items like dry box or headache rack.
This is also a function of how far the engine, transmission and ancilliaries protrude from the rear of the cab.
An example of the larger would be a mate's Isuzu FTR, which has a HUGE plastic intake tube going right up to the top of the cab. It has to be 6-7" diameter, which would necessitate 9-10" between the back of the actual cab sheet metal and the front of the box.
Some of the smaller trucks, like NPR, have "smushed" or ovalised tubing to reduce this gap to as little as 3" between BOC (back of cab) and mounted items like dry box or headache rack.
This is also a function of how far the engine, transmission and ancilliaries protrude from the rear of the cab.