Hey guys, I'm sure you've answered this 100 times. I know this isn't a hot rod question so I appoligize. I have a mildly built 454, I'd guess around 400hp. It is in a heavy 1 ton truck with a 5 speed manual. I use this truck to tow a good bit and just cruise around. I have a 750 holley vacuum secondary on it now. It does pretty good for cruising but when loaded I have to get the rpms up enough for the secondaries to open. I'd rather have a douple pumper so if I push the throttle enough I can open the secondaries at a lower rpm. I tried a really light spring in the secondaries but without the pump shot it makes it bog when they open. Every place I read says you don't want a double pumper for towing but no one says why. Would it be a bad idea to switch to a dp? Also would it kill my fuel mileage? I'm getting about 11mpg empty with the vs... pretty good i think for a 6600lb truck.