Originally Posted by carringb
I also took my dad's 5.4L van with my Weekend Warrior once (snapped off a pinion gear tooth on my van) and it burned quite a bit more fuel towing that heavy (~11,000 lbs) compared to my van.
Agree it can happen, but we should understand under what conditions and why.
In my opinion one data point doesn't paint a broad-enough picture unless it's put in right context based on well-documented and accepted theory. With limited data too many things could be different, like headwind, different terrain, etc...
I say this only because modern engine efficiencies between wide-open-throttle and optimum load isn't that much. If you noticed "quite a bit more fuel" consumption with your dad's 5.4 versus your 6.8 it was probably due to other factors not being equal. Perhaps your V10 could pull in overdrive and the 5.4 needed to pull in drive. That kind of difference could explain greater fuel consumption.
If everything else were equal (same gear, required HP, RPM, etc...), then the smaller V8 would likely be more efficient most of the time, and at worse slightly less efficient some of the time. I figure that if V10s can handle 30,000 pound trucks and RVs getting 6 to 8 MPG, then a 5.4 V8 should be more than enough for normal van use -- including occasional towing.