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Merlyn2220 · 10-12-2017, 01:02 PM

So here’s a summary of the “rear shell” and other changes since fall of 2016:

Larger rear tires with a 15.8% gear reduction = no change in fuel economy. Any gains were offset by the wider tread (225 vs 285) and resulting worse aerodynamics.
Removing the giant 3 point towing mirrors and replacing with 2003+ paddle style = 5% improvement in FE. Also drastically easier to draft and keep up speed on the highway.
Adding a driver’s side axle-axle bellypan = no change in FE, likely because you really need front-back to see a significant improvement.
Having a failing 25-30psi fuel pump = 10% better FE. I didn’t have an AFR meter at the time, but I bet it was running really lean.
Addition of a squared-off rear shell = 5% worse FE.
Completed driver’s side side skirts (with rear shell unchanged) = 10% improvement in FE
Addition of huge Class IV rear hitch = 5% worse FE
A/C use = 4.5% worse FE
Changing rear shell from squared off to ~12 degree taper = 5% improvement in FE
Driving ~60-65mph due to overheating = 7% better FE than driving 70-80mph
180F thermostat vs 195F thermostat = 2% worse, likely no significant change
Completely removed rear shell taper = within 1% of the 4” long 12 degree “short taper” but noticeably less stable in crosswinds and while drafting

I’m currently at about 14.8mpg and easily keeping up with 70-80mph traffic. Driving 60mph on the highway here is, like almost everywhere, extremely risky. I turn into a rolling roadblock that only causes problems for myself and other drivers who are expecting consistent traffic flow while texting. A ~7% improvement in FE is not worth risking my life on a daily basis, and it causes me to be unable to meet my delivery schedules.

Modifying the rear shape to add taper was spectacularly unsuccessful and a complete waste of time. The only thing I’ve concluded is that a “trip line” like used on the back of a Volt or Prius does seem to help highway stability. It’s especially noticeable when drafting a semi, where the alternating left-right vortexes coming off the semis push the van a LOT. I did get a chance to draft a semi with the “TrailerTails” installed, and it was quite a bit nicer on me, with a lot less buffeting.

A couple of other interesting conclusions:

Adding a big honkin’ hitch to the back, dangling out in free air is pretty noticeable.
Changing the thermostat from 195F (stock) to 180F made almost no difference.
Running lean due to a failing fuel pump is *probably* a significant improvement

One thing I noted is the consistency from tank-tank when I did a few back-back with no changes. Some people have said you have to run 2-3 tanks to be sure that the mileage is consistent. I’m filling a 35 gallon tank, so 1 of mine = 2 for most vehicles, and 3 for some like my Miata.

So unless I get really odd numbers due to traffic jams I’m going to go back to 1 tank = 1 mod.

10-12-2017, 01:02 PM	#36 (permalink)
Merlyn2220 Deep Lurker Join Date: Sep 2016 Location: Central Florida Posts: 53 Creepy Van - '95 Ford E150 Cargo 90 day: 14.45 mpg (US) Thanks: 5 Thanked 31 Times in 22 Posts	So here’s a summary of the “rear shell” and other changes since fall of 2016: Larger rear tires with a 15.8% gear reduction = no change in fuel economy. Any gains were offset by the wider tread (225 vs 285) and resulting worse aerodynamics. Removing the giant 3 point towing mirrors and replacing with 2003+ paddle style = 5% improvement in FE. Also drastically easier to draft and keep up speed on the highway. Adding a driver’s side axle-axle bellypan = no change in FE, likely because you really need front-back to see a significant improvement. Having a failing 25-30psi fuel pump = 10% better FE. I didn’t have an AFR meter at the time, but I bet it was running really lean. Addition of a squared-off rear shell = 5% worse FE. Completed driver’s side side skirts (with rear shell unchanged) = 10% improvement in FE Addition of huge Class IV rear hitch = 5% worse FE A/C use = 4.5% worse FE Changing rear shell from squared off to ~12 degree taper = 5% improvement in FE Driving ~60-65mph due to overheating = 7% better FE than driving 70-80mph 180F thermostat vs 195F thermostat = 2% worse, likely no significant change Completely removed rear shell taper = within 1% of the 4” long 12 degree “short taper” but noticeably less stable in crosswinds and while drafting I’m currently at about 14.8mpg and easily keeping up with 70-80mph traffic. Driving 60mph on the highway here is, like almost everywhere, extremely risky. I turn into a rolling roadblock that only causes problems for myself and other drivers who are expecting consistent traffic flow while texting. A ~7% improvement in FE is not worth risking my life on a daily basis, and it causes me to be unable to meet my delivery schedules. Modifying the rear shape to add taper was spectacularly unsuccessful and a complete waste of time. The only thing I’ve concluded is that a “trip line” like used on the back of a Volt or Prius does seem to help highway stability. It’s especially noticeable when drafting a semi, where the alternating left-right vortexes coming off the semis push the van a LOT. I did get a chance to draft a semi with the “TrailerTails” installed, and it was quite a bit nicer on me, with a lot less buffeting. A couple of other interesting conclusions: Adding a big honkin’ hitch to the back, dangling out in free air is pretty noticeable. Changing the thermostat from 195F (stock) to 180F made almost no difference. Running lean due to a failing fuel pump is probably a significant improvement One thing I noted is the consistency from tank-tank when I did a few back-back with no changes. Some people have said you have to run 2-3 tanks to be sure that the mileage is consistent. I’m filling a 35 gallon tank, so 1 of mine = 2 for most vehicles, and 3 for some like my Miata. So unless I get really odd numbers due to traffic jams I’m going to go back to 1 tank = 1 mod.