My 64 two ton GMC with a more modern 350 SB Chevy an SM420 and a two speed rear gets about 6 MPG and she is screaming at 55. I don't know for sure but chances are it gets better mileage with the 350 than it did with the stock 292. The SB is a little happier at the higher RPM than the original six. Oh and it seems too get about the same empty or loaded.
