[cal30_sniper]

If I was guessing, I'd say it's due to two factors.

The first is torque. The 454 makes a lot more torque than the 6.2L. With that kind of torque on tap, speed increase doesn't penalize the engine as much. An engine that's already working hard to get to 70 is going to lose a lot more mileage getting to 80 than an engine that isn't having to push as hard to get there. I noticed this with my 350 Suburban. 65mph would regularly net 17mpg. 70 was more like 16. Push it up to 80, and you were in the 13-14mpg range. I just don't notice the same sensitivity out of the 454, but it could be because gas mileage is relatively pretty bad to start with.

The second is altitude/terrain. I never get as good of mileage going west as I do coming back east, due to the simple fact that I have to climb so much on that stretch of road going east. You gain about 2000 ft of elevation between Austin and Ozona on I-10. From there further west, you gain quite a bit more. That starts dropping mileage fast in a vehicle this heavy, but it can be hard to track sometimes. At 14 mpg, a 10% decrease in gas mileage still puts you at 12.6mpg. Those differences can be hard to track if you don't take very careful notes at fill ups. I keep track of all mine on an iPad app. It lets me plot trends, etc. It also tells me that I only get 8-9mpg around town, haha.

The only other significant difference I can think of is direct drive vs overdrive. A 10 mph difference in direct drive results in a much higher RPM difference than a 10 mph difference in overdrive. The 454 almost never kicks out of overdrive, where I'm betting the 6.2L does quite frequently going up grades. That really adds up at higher speeds.

Going west, I get 13.0 mpg at 80. Coming back east, I get 14.2. That's over a 200 mile leg each way. Averaged out, you come out to 13.6mpg. The rest of the way east (when you hit flat country), I get 14.5-14.7 at 70-75mph. That's more like a 7.5% difference for a 9% speed decrease. I tend to push over the speed limit quite a bit more when it's 70 vs when it's 80. I don't go over 80 much at all.

I'd say the rest is in gearing and torque. Your diesel gets a lot better mileage to start with, but it's a lot more susceptible to speed and load changes because it's powerband is extremely narrow in comparison. The 454 gets bad mileage across the board, but it doesn't get much worse with moderate changes in speed and load because you are still operating well within its powerband, promoting better efficiency.

That's my stab at the topic, haha.

-cal30sniper