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Don't expect better mileage out of the bigger motor. IF you get 15mpg and keep it stock be very happy. IF you get close to 12 ( and by these numbers, generally highway and city hasn't mattered to my BBC's in the past) be happy. What is the motivation for the change?
Ok, I am looking at your sig, are we comparing 454 carb'd vs 454 TBI? If you are getting that mileage out of your current 454, you should be able to achieve the same with the TBI if the 15 your are getting is highway and not city... Emissions will be better in my opinion as the ECM will be keeping the engine in constant tune. Of course if you are happy with it overall...