On the left is a head with Military precups on a 1986 6.2, 3 bars. The cups sitting on the head are 1995 turbo precups, round dot. The head on the left is 1995 NA 6.5 precups, square mark.
I have run all three on a 1993 DB2 turned up w/ 4.10's towing same route and trailer. On a turbo application the 6.2 precups cost you 3 MPH at WOT full trailer load at 65 MPH. (The 6.2 precups run you out of air and turn more fuel to smoke regardless of boost the GM3 is at.) No difference with the 6.5 NA or turbo precups in my application: turbo. MPG is the same with all 3 precups shown. (NA as designed the 6.2 precups have the better economy towing or not.)
I have run all three on a 1993 DB2 turned up w/ 4.10's towing same route and trailer. On a turbo application the 6.2 precups cost you 3 MPH at WOT full trailer load at 65 MPH. (The 6.2 precups run you out of air and turn more fuel to smoke regardless of boost the GM3 is at.) No difference with the 6.5 NA or turbo precups in my application: turbo. MPG is the same with all 3 precups shown. (NA as designed the 6.2 precups have the better economy towing or not.)