How much fuel capacity do you gain from the vent mod?

 

Well, the tank on the short-bed can hold 40 gallons, filled to top of the filler tube; "gain" of 5-6 gallons. Don't know about the long bed tank, but probably similar gain.

 

It's not really extra capacity,but it just lets you fill it up to max faster. Normally you have to spend 10 min. trickleing the fuel in to get the extra space filled up, which on my 36 gall. tank I can actually put in about 40, If I had the mod I could get to that 40 mark in the time I get to the 36-37 mark.