A properly selected and tuned multiple carb setup on a street vehicle will IMPROVE fuel economy given reasonable driving habits.
Jon, I'll concede that a perfectly-tuned and properly-engineered multi-carb system can do wonders for performance, and at least equal, if not improve gas mileage. From the discussions we have here on a regular basis, I'm prone to believe that finding that mythical combination of just-right parts selection and correct tuning, and then keeping it in tune, is about as likely as finding a chicken with lips!
At various times and places, I've been the only guy in town willing and/or able to take on multi-carb setups, and make them run right, but I'm firmly convinced a good single-carb system is preferrable, especially for a daily driver that doesn't spend most of its time parked at a car show trying to impress onlookers with that sexy multi-carb system.
So far, that approach has worked on Austin-Healey, Jaguar, M.G., Honda, Triumph (car and motorcycle) Chris-Craft boats, Ford, Pontiac, Chevrolet, and a few other vehicles, with Zenith, Solex, Mikuni, Stromberg, Weber, Carter, Holley, Edelbrock, and Predator carbs.
Jerry