At 68 (ish) mph in OD with an empty 20 foot trailer I got an average of 10-12 mpg on a 165 mile drive all on highways. On that same route and weather conditions fully loaded (box and trailer) rolling at 60 mph in 3rd I got an average of 12 to 14 mpg. How is that possible? No friggin idea but my theory was since the engine rpms are running around 2500 the motor used that velocity and wasn't throwing near as much fuel at each cylinder. At 55 mph my truck I think could get 14 or so but nothing much better than that. The higher the speed the worse the mpg is. Running at 75 mph my truck gets like 10 mpg.

 

That's nuts, I have no idea how that would work.

 

To those that say just get a fuel efficient vehicle and don't worry about the trucks gas mileage. I hear the same thing about classic cars etc. That makes no sense. You would still want it to run well and get the best economy it can. My 65 Mustang gets 20 mpg sometimes better on the highway and that makes it so I can actually drive it. If it got 10 like some other classic cars, I'd drive it much less or on only shorter drives.

If you want the best mileage keep it stock, that is what the engineers were trying to achieve originally. Few things other than maybe regearing will help mileage.

Best advice is maintenance, fluids, filters, etc. And of course not driving fast, hard, or excessively braking.

These trucks are old now and most won't get as good as they got new. Also, the ethanol fuel likely reduces it a bit as well. I have daily driven these trucks since they were new and still do. I noticed with that switch in fuel there was a 1-2 mpg drop on average that I never saw again.

My 94 F150 2wd 5.0 4R70W with 3.73 gets about 12-14 and maybe 15 on a good day out on the road. My previous truck with identical drivetrain used to get 13-15 and sometimes 16-17 out on the road, but it also went down to the same as this truck around the same time gas changed.

Avoid any gimmick parts like multi electrode spark plugs, hot air intakes, Tornado etc.

Find a 300/460 intake tube at the junkyard but keep a normal air filter.

Don't run big tires especially larger than 31 10.50.

I don't think its as big of a deal with the 5.8 but lugging the 5.0 around in a higher gear unnecessarily will get worse mileage. I leave my OD turned off every time I drive in town or on slower county roads etc. Only turn OD on when on the highway. That will also save the transmission from excessive wear hunting gears.

 

Did you change from an E4OD?

 

There is no way I'd convert a nice simple easily maintainable and repairable vehicle to a convoluted hybrid making it no longer a nice simple vehicle. This is why there is not a newer car with tech junk anywhere near my house.

 

This truck and my previous truck were factory with 5.0 4R70W. It is common on 94-96 5.0 trucks with MAF.

 

That better mileage loaded must be from the weight of the load pushing the truck, except on uphill pulls. Interesting.
On certain runs, a manual transmission in an 18 wheeler, will get better fuel mileage than an automatic.
The automatics shift way more than a truck driver driving a manual.
My truck makes peak torque at 2400 RPM, so I guess by staying near that number, it would pull weight more efficiently.

 

Wind resistance starts having a major impact at around 50+ mph. Wide tires have larger rolling resistance. You need to treat the skinny pedal like a newborn baby, gental. Then maybe you'll get better mpg. I wish I got 14. My 460 likes fuel.

 

Fuel delivery with EFI is heavily based on engine load, RPM factors in too but overall if can operate at a slightly higher rpm at a lighter load then it will get better fuel milage than lugging it at lower rpms.

 

So just a thought here, would that mean possibly keeping the rpms in that 2500 range would help with fuel economy? Just curious. It kind of surprised me myself that I got better fuel economy on that trip but I mean the only time I was leaning into it was the inclines otherwise I had the throttle pushed in maybe an 8th of the way tops on the flats.

 

Yes, basically the highest speed one can maintain with the least throttle input(highest vacuum) will result in the best possible fuel milage. Everything factors into this... weather conditions especially wind, terrain, vehicle aerodynamics, vehicle weight, engine displacement, tires size, trans and axle ratios.
In this case using D instead of OD puts the engine into an rpm range where it makes more torque so it has an increased load range and uses a lower percentage of total to do the same work, and there is more mechanical advantage with that drivetrain gear ratio which directly reduces engine load. When combined these two things result in lower fuel consumption at that road speed.

 

Exactly right, vehicle efficiency is key. Why I never use OD in the city as it drops my RPM too far down where I have essentially no power to operate. Plus the SOB just shuffles between TCC lock/unlock and OD application.

Look at Uhaul box trucks, they used to come with vac gauges. My W126 Mercedes have them factory installed.

As pointed out, drag increases exponentially with road speed. Going 70mph versus 35mph has four times (4x) greater drag on the vehicle. Mix in the poor CD these have, inefficient engines in stock form, and lack of axle gears (in half tons at least)...you pretty much are stuck.

 

Very interesting. Thanks for the response. Sorry hjartland if you felt I hijacked the thread there for a second but these guys are pretty damn smart so maybe just try driving without overdrive at 60 mph, it might be worth your while. Pretty interesting that a higher rpm would help with fuel economy but I too feel like OD drags the engine down too far. Usually on the steep hills I have to lean into the throttle to get up the hill in OD but slowing that down to 60 mph in 3rd I would put half throttle at most empty anyway with no trailer. On the wonderful windy days in Kansas I usually have to go into 3rd if I'm going into the wind because the engine just doesn't have the git up to push it.

 

Not at all. This has been an interesting conversation.

 

If you don't care about cost for the increase, then gt40p's, a set of long tubes would help you out......if, of course, you stay gentle on the throttle after the swap.