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TheAvo has it right, or at least his uncle did nearly 30 years ago. Trucks have big engines designed for power and not for economy. Trucks are heavy - they have to have stout frames and suspensions to tow and haul, and must have larger and heavier engines to match. Trucks are inherently non-aerodynamic. The combination of these factors guarantees inferior fuel economy. That's the trade-off. For capability you trade economy.
That's for the typical truck - any truck. But this is a forum that exclusively pertains to one specific truck - a full-sized truck with very powerful engines that is specifically designed and equipped for high speed off-road use. More trade-offs obtain, of course - even more capability for even less fuel economy.
If you've read this far you're probably saying, "Yeah, that's obvious." And I'd agree with you except that this thread is 8 years old, is 164 pages long, and is one of the most highly visited threads in this forum. Quite evidently it's not obvious at all to a great many Raptor owners.