The late Bruce Plecan "Grumpy" on TGO was getting 17 mpg out of an earlier 454 TBI that had 4.10s and 31" tall tires and 12-13 towing. The tuning in that Suburban had some ideas he had implemented on his truck. At light load we were targeting very low 02 switch points with a fair chunk of cruise timing. Made the engines run smooth and crisp.
I'm still skeptical. I've been a member on TGO for over 20 years, went back and looked, only saw his claims for 14-15mpg and on a 1000mi trip where he was babying for mileage.
My 454 'burb has done a best of just over 14mpg twice(and I was beyond excited, 11.2-11.5mpg back and forth to work DD is normal for this rig). Both times were keeping it in the 68mph range, and involved no wind/tailwind scenarios. I don't drive aggressive, and it has been tuned, has better exhaust, HT tires, upgraded injectors, big 3, etc. Granted I'm 4wd, but a GMT400 is slighty more aero(this is almost comical to say when discussing a brick) than a squarebody and mine has overdrive.
I don't doubt you and Grumpy got better than factory mileage out of your rigs...but I'd have to see the odo and gas slips to believe those numbers were normal or average. Single trips in the right conditions with an egg under the gas pedal, it becomes more believable.
OP sorry for more cluttering of your thread!