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2015 350 XLT with 3.7 L/3.73 rear.. horrible mileage

13300 Views 27 Replies 14 Participants Last post by  ranxerox
First of all, we do live in WV so there are some hills around here, so I expect *some* decrease from the 19 mpg highway estimate. However, I'm getting a hand-calculated 13.7-14.2 around here most of the time with an 80/20 split of highway/city, respectively.

Van has 35,500 miles on it and the only thing that's not stock is the tires... currently running 225/75/16's instead of the 235/65/16's, so it's a 4% increase in tire size... that only equates to around 85 rpm at 70 mph, so it's not much. I had to change them out after we'd had the van less than 2 weeks because the stock tires started showing cords and throwing chunks of rubber off of them at 34,500 miles.

Anyway, I talked to a guy from church today who has basically the exact same van as us with the same amount of people to haul (and drives the same route to get home), and he's getting 17-17.5 average, so around 20% better mileage. I have no idea what's going on with this thing.. There's no CEL showing, so it doesn't appear to be something so big the computer is fussing about it, but I can't imagine that mileage being anywhere near normal. If I keep it at 45 mph around here on the side-roads I can get a decent mileage reading on it if I reset it and run a few miles, but once we hit the highways and actually have to GO anywhere, the mileage drops like a rock. I don't ever even push it past 72 or so, either, so it's not like I'm running it hard.

Any ideas? Going to try to get it into the dealer to run a diagnostics on it for the awful mileage this week, so hopefully that will shed some light on it. I'm just about ready to trade the frustrating thing in for an ecoboosted van, though, as at least those have a decent amount of torque to get up the hills.
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I do think the hills around here seem to clobber it when running around with the cruise control on, too. It'll slow down 4-5 mph and then downshift all the way to fourth after a very short time in fifth. You can practically watch the fuel gauge go down at that point.
The biggest difference between the two engines is described with your quote above. That is the 3.7.

The 3.5 will not downshift and will hold the speed it's set at. I've only seen my 3.5 downshift towing my trailer loaded with 3-4K lbs in the trailer.

As for fuel economy, when the ecoboost is boosting I don't think it's efficient. I'm 17 mpg light highway. But loaded it's 13 or so. City is 15.

But I like the fact it doesn't have to downshift. Which while people think the turbo's are added complexity, the transmission on a 3.7 van is going to fail a lot sooner than a 3.5 trans that stays in lock up most of the time.
I can understand a drop in fuel economy with AC running with city driving. But highway MPG driving should not really be effected by the AC. It should be a marginal difference, or negligible. But yah in the city idling around it would be a different story.

I certainly had slightly better expectations for fuel economy than I see. My 5.0 V8 F150 4x4 wasn't worse, seems about the same. In fact I think the v8 was better on the highway. And my F150 was a super crew long bed, 4x4.

I think I'd actually like to see them put the v8 5.0 into the transit van. I actually would have to try driving that and would consider it over the EB.
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