Fuel mileage is hard to say. Didn't keep track as it was a dedicated wheeler. I would say mpg would have been equivalent to my thermoquad. My speed demon carb (with spring loaded needles/seats) was definitely bad for fuel economy, but it sure did go!
Guess what I'm saying is, depends on what carb you have on now. I know the "off road carbs" are notoriously terrible on fuel, where a properly tuned thermoquad can be exceptional.
I wouldn't expect any mpg change.