Anyone with a 4.6 out there . . .

[David Harris]

On my D1, I believe the mpgs rose from about 9.5 in expedition gear (33s, fully loaded roof rack, bumper, at GVW) to about 11 or so when I went from a 3.9 to the 4.6.

I think the fuel economy increase was largely because the smaller engine had to work so much harder, especially going over the mountain passes here in Colorado.

I had my relatively stock Disco (31's, that's it) out there this Summer. Going over Wolf Creek, I was down to about 20-25 mph at the top. My vehicle was loaded down pretty heavily, probably about 6000 lbs. total. My 4.0 did pretty well, just slow. I was averaging about 12-13 mpg on that trip.

 

[Dave Legacy]

That is interesting. My LR3 puts my D1 to shame... my D1 was getting 14+mpg (on 33's) on a recent road trip. My LR3 got close to 20 with the 285/60's.

I took a trip with some other D1's and D2's and the D2's (4.6L) were getting 2+ mpg compared to the D1's. All the vehicles were comparably equipped - tires 33-35's, stock to 4.11 gearing, all loaded with gear.

I also find it odd, but I'm finding that my highway mileage is really good and it's just my city mileage that is terrible. In fairness I haven't had this truck long enough to collect great data and I realized that my first couple tanks I only had 30psi in my rear tires.

 

[R_Lefebvre]

Dave, that's really not that surprising. Under heavy load, the larger engine is more efficient because it can operate in closed loop mode. But at light loads, idling, etc, the larger displacement is just wasting more fuel as it has more friction, more thermal losses, etc.

 

[craig]

I also find it odd, but I'm finding that my highway mileage is really good and it's just my city mileage that is terrible. In fairness I haven't had this truck long enough to collect great data and I realized that my first couple tanks I only had 30psi in my rear tires.

Yes, a moderately larger diameter tire should increase your MPG on the hwy. Larger tires travel further on each rotation. It takes more power (gas) to get them moving, but once moving they can be more efficient. It's like running down the highway in a higher gear.