Snakebite202, What they're rated at and what they actually get are two different things. The AUTOs got about that MPG, but the manuals get WAY better... You can take my word or you can ask anyone with a manual C5.
2fast4theLaw likely had an auto. Those sound like Auto numbers to me... In fact, I had an auto as well and that's what I got until I put 3.73's in the rear end where I lost about 1-2 MPG. All my friends and I get/got beyond 30 on the freeway with the m6. The BIG difference between the 4L60E (auto) and the T56 (manual) is that the auto had a crappy overdrive after 3rd whereas the T56 has 2 overdrive gears (one for street, one for freeway) and at any time, the car can cruise at 1500RPM. 70MPH in 6th gear on the freeway is 1700RPM.
The LS1 is already a very efficient motor... Giving it better gearing like that is only going to help its case. The auto, however was a 3 speed + over drive. And that trans was terrible for fuel economy.
LS1 Gas Mileage - LS1TECH <-- Totally unbiased MPG results. Some really low (from VERY modded cars) and some exceeding my expectations.
What shows up on paper isn't always true. GM under rated the LS1 F-Bodies to sell more Corvettes. So are you saying that just because it's on paper, the F-Bodies were 325HP instead of 350? Because a dyno would say otherwise.
And yes, that saying is old. But I don't feel it's outdated. Technology only gets better, but the truth behind the saying is still what it is. A current day Big block is still FAR better than a current day small liter motor. ANYTHING you do to the smaller motor, you can do to the bigger motor and the results are only escalated, but not changed.