1986 chevrolet silverado C-10

Looking for original specs on truck, Engine has been changed from a 305 to 350 with a holley 4 barrel carb. Wondering what feul mileage i should be getting.
Basically any tech stuff on my truck would be nice .
Thank you
pat

My dads old 85 Silv 4x4 used to get 10-13. It had the 350 in it, but it also had a 4" lift and 33x 11.50s on it. I'd say you could expect 15 tops.

15mpg sounds about right to me although it really depends on how often you kick in the secondaries.