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Old 05-22-2014, 08:37 AM   #1
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Saving fuel in a tow truck

The Truck:

1992 Ford F-450 (F-Super Duty on the fender) dually 18 foot rollback
7.3 liter (444cid) non-turbo (IDI) diesel V-8 engine (International/Navistar)
5-speed ZF manual transmission (close ratio, 0.77 or 23% overdrive ratio)
5.13:1 rear axle ratio (sick, isn't it?)
LT235/85R16 mud and snow tires (definitely not a highway tread)

The Driving Habits:

In town, this truck is a pig. No turbo to help with acceleration, it's heavy, and it's geared very short so you have to shift up early and often. Whereas in a normal car or pickup truck to get to 40mph you shift like this:

ooooone...twoooooooo...threeeeeee...foooouuuuuuuuu urrr...fiiiiiiiiiveeee

In this truck you shift like this:

one...two...threee...fouuuuur...fiiiiiiiiiiiiiiiii iiiiiiiiiiiiiiiiiveeee

When I'm in a hurry I shift at 5, 12, 21, and 30 mph. When I'm not in a hurry I might be in 5th gear before 20mph, especially if there is even the slightest of downhill grades. I also don't MASH the pedal down. It's a diesel, it has torque, it doesn't need a lot of pedal unless you're pulling a hill or merging on the highway. When I am in that situation I may hold third to 35mph and hold fourth to about 50mph. I won't go much higher because the engine redlines at 3750rpm and the yellow part of the tach is at 3300rpm so I won't exceed 3300rpm in any gear. That's 61mph in fourth and 79mph in fifth, and I like a nice cushion.

If this truck is driven very hard (standing on it light to light, lots of pedal, cruise above 60mph) the driver can expect to get about 8mpg. At $4/gallon for diesel (it's been cheap so far this year, sometimes 4.09, but usually can find under 4) that means fuel costs 50 cents per mile or so. Ouch. This is how the PREVIOUS driver of this truck drove it and he warned me about the 8mpg thing. I knew it could do better.

If this truck is driven with the flow of traffic (not drag racing or anything but not gently), cruise at no more than 65mph empty and 60mph loaded, the driver can expect 9mpg in town and 10mpg on the highway. This is how I drove the truck to begin with.

If this truck is driven gently, slower than the prevailing traffic flow, no hard acceleration, cruise at no more than 55mph empty or loaded and limited to 50mph if loaded extra heavy (vehicles both on deck and wheel lift, or a 6000lb SUV/van/truck on deck), the truck will return about 11mpg in mixed driving, and doesn't seem to care if it's in town or on the road, it'll get 11mpg. This is how I've been driving the truck most of the time since being put in it.

If this truck is driven like a herd of turtles, slower than all traffic flow, accelerating gently to where overweight dump trucks would flip me off and grandmothers could angrily pass me on the shoulder, cruising at no more then 45mph, the driver can expect to get 13mpg. I learned this when it was icy and snowy out and I could NOT exceed that speed or accelerate hard on bald tires without losing control. Tires have been replaced since, but 13mpg is what I got eight tanks in a row during bad weather. The truck IS CAPABLE of this mileage in its awful condition.

My goal, be able to get that 13mpg ALL THE TIME or maybe even better.

I am limited by that rear axle ratio, the size of the engine, the tread on the tires, and the general weight and shape of the tow truck.

A few changes in driving habits allowed me to come very close to this. I took a day of steady work and chose to shift earlier than normal, accelerate slower, and not exceed 52mph no matter what. I was able to get 12.45mpg on both of the two tanks of fuel that day. But if I work slower, no amount of fuel savings will make up for the reduced number of calls I'd be able to run. To wit:

This truck is driven 180 miles per day most days. Sometimes more, rarely less. On those days I can run about 12 calls and earn around $300. That means at 11mpg driving like I am now used to driving, it consumes about 16.36 gallons per day, about $66. If it was getting 13mpg it would consume about 13.84 gallons per day, about $56. That is a $10 a day savings. But if I am only able to run 11 calls due to running slower, and I make $275, I save $10 but lose $25 so I lose $15 overall. The happy medium is what I'm looking for. That 12.45mpg the one day would be nice to get every day.

So I mentioned earlier I am limited by that rear gear ratio. That MIGHT be able to change. Problem is on the Ford Super Duties from 1987 to 1997 they used a specific carrier on the Dana 80 axles out back and it only fits 5.13 or 4.63 ratio ring and pinions. The carrier would have to be changed out to go lower numerically. Dana 80 ratios are available in 3.23, 3.31, 3.54, 3.73, 4.10, 4.11, 4.30, 4.63, 4.88, 5.13, & 5.38. But not all of those are available for the 37 spline axles. 3.23 I think is the one you can't get. So ideally, for best fuel economy and efficiency, this engine should run around 1800-2000rpm when well loaded and about 1600rpm when empty or lightly loaded. I don't mind running slower loaded so I wish it was the other way around. What I need to do is figure how much higher of a speed this truck can travel before it suffers aerodynamically so much that a gear change will cause it to get LOWER fuel mileage.

Anyway, I'm rambling again, but as things are now I would LOVE to be able to run 60mph most of the time at 1800rpm. That means empty I can slow down a bit to 54mph and enjoy better mileage at 1600rpm, and when loaded and in a hurry I can go 2000rpm cruising at 66mph. To do this requires a 3.73 ratio. If I split the difference and go with 4.10/4.11 gears, the 1600rpm would be at 48mph (too low for the highway) and 2000rpm would be right at 60mph.

For reference, this truck runs over 2700rpm at 60mph as it is now.

This is a lot of numbers, I'll post more later. Hoping they can get the gear change done soon.

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Old 05-22-2014, 10:44 AM   #2
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Yeah, the gearing sounds like the big bottleneck there.

For what it's worth, LT235/85R16 is a common and relatively inexpensive size and is reasonably easily found used on Craigslist in a variety of highway tread designs for very little money, even in load range E. LT245/75R16 is the same diameter and is even more common, with more highway tread designs (still with load range E). However, you could also go up to a taller tire, which would help with that gearing issue. I assume you're already running 80psi?

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Old 05-22-2014, 11:23 AM   #3
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Actually the 235/85 tire is 31.5 to 31.7 inches and it matches up better to the 265/75, not 245/75 (which is more like 30 inches). I have 265/75 tires on my Suburban, Michelin LTX MS2's and they're great on the highway. What's on the rollback right now are these cheap little bastiges:

The 265/75 version of that tire has a higher load rating (3415 per tire instead of 3042). Only problem is with a tire that wide in a dually application, they would rub on each other. Even at full 80psi, with a 5000-6000lb vehicle on the bed, they would squish down and touch. I think I would end up looking at a more highway-rated tire and save these Courser LTR's for winter use, if I was the one buying tires.

Of course for buying my own rollback, I would absolutely get highway rib tires and then have something less slippery for winter use. Mastercraft Courser HTR would prbably work OK but I'm thinking more like a Michelin XPS or Toyo M54/M55 sort of tire. Then again some of the trucks I'm looking at are 19.5 inch wheels which simplifies that problem as far as tires (just makes it more expensive)
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Old 05-22-2014, 11:52 AM   #4
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It's always interesting to see how technology has moved on in recent decades. It's weird to think this truck uses almost 1000% more fuel than a modern diesel car!
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