gas mileage Mechanical vs Vacuum Secondaries

A lot of good info and tips in this thread, but to answer the original question. If tuned correctly and depending on how your engine is built and how you drive, a vacuum secondary can get better mileage than a DP, but it won't be that noticeable unless you have a 4spd which would be a considerable difference since you are basically accellerating 4 times for every one of the auto and with the dual accelerator pumps = "a lot of gas".

To answer someones question above, "why run a DP if your not racing a lot" it all depends on your engine, a vacuum secondary carb will not work, or at least work correctly if you are running upwards of 300 duration.