Since I track my mileage every tanks via my phone I decided to do an experiment. I had read a number of posts by people with similar vehicles that stated the best way to increase mileage is to slow down. I did this and here are the numbers I found....

Vehicle: 1988 E250 7.3 IDI NA, 3.54 gears, c6+Doug Nash O/D

Procedure: I tracked my mileage for 4 tanks during my normal weekly commute. My commute is 60% highway and 40% city. Terrain is slow rolling hills. For each tank I set my cruise when I was on the highway for a different speed. I started at 75-80mph and dropped each tank in 5mph increments. I filled up at the same gas station on the same pump for each tank. For consistency I also put the DN in over drive and just left it. This made it pretty doggy around town since it gave me the equivalent of having 2.83 gears.

Tank #1, 75-80mph: I drove 387 miles using 27.851 gallons of fuel. Netting 13.90mpg

Tank #2, 70mph: I drove 425 miles using 27.784 gallons of fuel. Netting 15.30mpg

Tank #3, 65mph: I drove 452 miles using 28.733 gallons of fuel. Netting 15.73mpg. This tank could be slightly skewed. I was having charging issues so I let it idle during stops instead of risking it not starting due to a low battery. Idle time was about 1 hr total. Some brief searching said I would use about .5 gallons an hour at idle. Removing this would give a mileage number of 16.01mpg. It was also raining consistently throughout this week.

Tank #4, 60mph: I drove 464 miles using 26.898 gallons of fuel. Netting 17.25mpg. I did accidentally set the cruise to 65mph for one 25 mile highway trip, but this would only effect the results by about .05mpg.

Conclusion: While there are a ton of variables and one tank sampling per speed does not provide the most accurate results, it does seem to show that reducing your speed does seem to have a fairly large positive affect on mileage. Mileage showed a 24% increase between tank #1 and tank #4.

Other thoughts: I would be curious to see if dropping to 55mph would continue the trend, unfortunately I couldn't bring myself to slow down that much. Our speed limit in the area is 70mph with most people going 75-80mph. Going that much slower than the traffic flow would drive me crazy. I also wonder if having the OD engaged during city driving had a negative affect on my numbers. It definitely took more throttle input to get up to speed and my EGT seemed to be higher doing it vs with the OD off. Once cruising my EGT were significantly lower with the OD on. I also did not check my tire pressures, but since I just had them rotated and balanced I would assume they set them to the recommended stock pressure. Increasing them to 80psi may reduce rolling resistance and net a small gain.

 

I've long suspected that the best mileage would be achieved at the lowest speed at which a vehicle (with auto tranny) will hold its top gear--which is around 40 MPH usually. However, roads with a 40 MPH speed limit are usually littered with stop signs and stop lights, so there's not a good place to test that (it would probably get you shot if you tried it on the interstate!)

Jason

 

Yeah, there were a few not so happy drivers when I was cruising at 60mph in a 70mph zone.....oh well.

Lower speeds can help a lot since power needed to increase your speed due to wind resistance increases exponentially rather than linearly. The only issue you run into with slow speeds and high gears is that it can put you well below your torque peak. This can mean that it would take more throttle input to maintain your speed. Less revolutions, but more fuel per revolution.

 

You don't need to be at your engine's torque peak to cruise--you only need to be at an engine speed on the curve where the available torque is greater than the torque needed to maintain speed for a given throttle input.

Jason

 

You are correct. If you can maintain the speed without having to increase your throttle input then you would should get better mileage.

I guess I was looking a little more at reducing the engines rpm at a given speed. There are examples of individuals who's fuel consumption does not improve or gets worst with reduced RPMs due to the fact it is too far out of it's power band. This can be seen with an EGT probe....the temp will be higher while the RPMs are lower.

 

Try rolling up to stop lights in a slow coast. Lift off, coast into lights, stops. Moving from stop uses more fuel than moving faster from a roll.


Tire pressures are important, tread design, rubber hardness.
Mirrors ?


Do you feel a 'hull speed' for your truck? A speed where truck feels comfortable ? If you run below hull speed then mileage should increase.


If the truck is always run below 'hull speed' with quality motor oil changes, warmups...the engine lasts 2X running faster.


I like a hull speed running with a Santa Ana rearward over the mtns east of San Diego. Up past El Cajon. Not often....

 

That's great: I love it. I have done this same experiment with my V10 E350.

The air resistance and coasting and idling are huge factors in a work van. I do about 60-80 miles strictly highway each day and another 60-80 mixed city/highway.

Whenever I resolve to "not exceed 65mph" I always get high 12's to 13's (mpg). I started doing this once I calculated: 65mph versus 75mph on a 40 mile trip is about 5minutes lost, but also about 1mpg lost.

Also I've found that coasting is very satisfying and efficient.

Idling is tougher to minimize here in Florida: it's 80+ degrees and sunny for 10months per year.

I want to get a Lie-o-meter from another van and install it in mine...

Also i am due for new tires soon and I've been wondering if it would increase my MPG by getting tires a little bigger or smaller.

 

There are more factors that affect mileage than one can count. Two more are wind speed and direction,(head wind, tail wind, cross wind) then there's water on the road surface (rain which was already alluded to) Diesels seem to get better mileage when you keep your foot out of it and feather the throttle going up and downhills. Listening to the turbo helps, keep the boost at or below 10 psi is where most will get the best mileage. I'm another who does the "coasting" thing too. it's easier to do with a manual trans though by just popping it outta gear on the downhills.

 

We just made a trip up to Eureka Springs Ar. this past weekend in the van. First fuel up was in the 13's filling in N La. the next was 15.4 at Greenbriar Ar (N of Conway) next was 12.3 (in E.Sprgs), last was 16.4 (S'Port La.) The first fill included speeds of up to 75 mph for the 1st 100 miles. The 12.3, the speeds never exceeded 65. Neither were the 15-16 fillups. The 12.3 was using 89 octane fuel, the 15 and 16 were on 89 and 87 respectively. Sometime this winter I'm going to advance the cam timing to see if that moves the powerband down in the rpm range. But overall, nothing blantantly obvious other than speed seems to affect the mileage with the roller 5.8

 

Great post. Even with a small vehicle like my Subaru Forester, I get 25 mpg when going my usual 70-80 mph, and once did a long leg of a trip keeping it under 65 mph and it got 31 mpg. (Both based on the dash lie-o-meter which I assume would be similarly inaccurate at either of these speeds.) It will get 33 mpg at about 45 mph at a steady cruise in 5th gear (it is a stick). I put on low rolling resistance tires and my 25 mpg looks to have increased to 26 or 26.5.

My E150 with 4.6 gets about 17 mpg on the road at my usual 70-75 mph cruise on summer fuel, but if I keep it below 65, it will do almost 19 mpg.

Knowing this, I still drive 70+ because I hate traffic backing up behind me, and am usually interested in getting somewhere. So yeah, speed matters a lot when you have a large frontal area to push thru the wind.

As for tires, if your gearing spins the engine too fast (overgeared for the kinds of loads you carry) then a slight increase in tire size should improve mileage, but will hurt city mileage if the tires are heavier or chunkier. I am guessing they don't make specific "low rolling resistance" tires in D or E rated tires, but Michelin LTX M/S's are known to have lower rolling resistance than many similar tires.

Good luck,
George

 

I have run a couple of more tanks through since the initial test.

2 of them I ran at 60mph on the highway portion of my commute (60%hwy/40%city). Both tanks registered a little over 17mpg (17.05 and 17.25).

Another one I ran at 60, 70 and 75mph. 70 and 75mph were both about 35% of the miles and 60mph was about 30%. Got 16mpg with it.

Getting ready for another decently long highway trip. Probably 6 hours round trip.

They make LRR tires for my van, just not sure how much they could help. I went around and added air to all my tires so they are at their max cold reading (80psi). Hoping this might be good for a little bit.

With things starting to get colder here I am sure mileage is going to start dropping due to winter fuel, extended idle time and snow covered roads. Maybe next summer I will get a chance to do a thorough test on my front air dam.

I asked the person I bought the van from about mileage. He said he didn't check it much, but when he did it was usually around 13mpg mixed driving. He used it to pull his 7k pound travel trailer and said he never got below 10mpg while towing. We are planning on starting to tow a travel trailer with it next year. Maybe I will get lucky and be able to get around 13mpg.....now I just need to sell my wife on the benefits of a turbo kit for it!!!!

 

Going to a taller tire does not always yield better highway mileage, it would depend on the engine and where it's the most efficient in the rpm band. I went to shorter tires on my van, to both lower it slightly and to increase the torque output by changing the effective rear gear ratio to a higher number. This didn't hurt it's mileage at all. The 1" change in tire diameter shorter tire)changed the 3.55's out back to a 3.67 ratio at the pavement. This complimented the GT40 heads and the 1.7 Cobra rockers, that had bumped the power up a couple hundred rpms in the rpm band. We had made a trip to California last December with 29" tires on it (it's got 28's now) the fuel mileage didn't change with the shorter, wider tires.

 

I personally think shorter tires look really dorky on the billboard-sized side of a van....I dislike the "pumpkin on a roller skate" look. But you can't go that tall on a van anyway--even the van stock tires are quite small compared to what they're putting on pickups these days.

You are right about the mileage as tying into engine RPM. The OP has a diesel with a 3.54 gear and an OD and it depends on how his gearing matches his loads and anticipated speed. If he runs lightly loaded, more gearing either thru tire size or an actual gear change *might* make a difference but the heavier tires would move mileage to the negative side.

I had an '86 GMC 2500 van with a wimpy carbureted 305 and a Turbo 350 with lockup. With the stock rear gear of 2.73, it was helpless on even a mild uphill. I changed the rear gear to a 3.42 (and added a limited slip axle) and the effect on both my city and freeway mileage was zero. And the performance improvement was astonishing.

It's not about the tallest gearing, but the right gearing for your engine and load.

George

 

Well there's nothing "dorky" about the tires or sizes I chose. The fronts are 235/70-15 mounted on 15 x 7 aluminum slots. The rears are 275/60-15 mounted on 15 x 10 slots. Both front and rear "fill out" the fender flares the van came with, instead of the dorky stock wheels which put the tires too far inside the flares. Now that was dorky, made you wonder why the flares were added in the first place.

 

Everyone is unquestionably entitled to their own esthetics. If you are talking about conversion van running boards with big flares, those are dorky too. I've got a conversion with very minimal, rounded running boards that fit into the body lines. Thin eyebrow over the wheel wells actually looks OK and adds almost no width.

255/70x15" is the traditional choice for big vans--same height as a 235/75 but wider. I had them on the three vans I owned before my current '02 E150, which had always had 235/75 Michelin LTX M/S's (on my 3rd set). IMO these are the best tires for function on a big van, ever.

I would like to go with 30x9.5x15 and tried once, lost 2 mpg, and took 'em back under a 30 day satisfaction warranty. Looked great, though. Actually you could get 31x10.50's on the rear. It would look cool with BFG AT's but lose a lot of mpg due to overgearing and tire weight.

Your flares may limit the height of the tires you use esp in the front, and like I said, you are as entitled to your esthetic as I am to mine. That's why we don't all drive the same vehicle.

Have a great weekend,
George