Ok, Just when I had given up on any form of mileage on the 03 expy, This happens.

On a recent trip from Houston to Dallas, I set the cruise on 74-75. No AC on, no headwind. Just me and about 100 pounds of gear. When I checked the mileage manually, I got 15.6.

On the return trip, I needed to make it home a little quicker. Same load & conditions, but this time I ran between 80-85 and hit 90 for a while before the cops got to heavy. I was figuring the mileage would suck. it was 17.1.

Can anyone explain how a truck can turn higher rpms and get better gas mileage? This is the fastest I've ran the truck and its the best road mileage I've ever gotten.

 

Where did you buy your gas? The gas sold in the major metro areas in Texas have so many crappy additives in them that really adversly affect gas mileage. Whenever I drive back to the DFW area from Houston I always stop in Waco and gas up. The mileage I get with that tank is always 1.5 to 3 MPG better.

 

It has to do with the torque peak. At 85mph you are turning about 2700rpm which is close to the top of the torque range, that is if you have a 4R100 trany and V-10, but as the motor is making its max torque it is not working as hard as when it is turning fewer rpm. I know it sounds counterintuitive but it is the reason.

 

The gas was purchased at shell both times (they seem to be the most consistent around here).

The motor was actually turning about 2300 and it is a 5.4l, not the v-10. I thought that the torque/hp curve might be the reason, but it's nice to have someone verify it.

It's not fair, to get decent mileage, I have to break the law, excessivley. Thanks again Ford.

 

Increasing the speed 10-15 mph increases the drag a huge amount (Drag increases are not linear; drag increases at the square of speed). Increasing the speed by that much should not increase mileage unless you're at low speeds. (I.e driving at 35 causes the truck to be in OD with the torque converter unlocked. Speed up to 45 and the converter locks, resulting in greater driveline efficiency--and the drag difference between 35 and 45 isn't sufficiently bad to override the substantial efficiency increase).

The torque curve idea is a neat one, but is inapplicable. Being at the peak of the torque curve doesn't necessarily mean being at peak efficiency (look at the torque/hp/efficiency charts for large engines (tractor, semi, etc.), peak efficiency is usually achieved below peak torque). The torque peak is the RPM at which the engine generates maximum torque under certain conditions. Maximum torque is not always generated at that speed. The difference is simple. If torque peak is 2500 rpms, and I sit in my idling truck in neutral and increase RPMs to 2500, a certain amount of fuel is burned to make the truck engine spin faster. The engine is not automatically producing max torque, however. It is simply spinning faster. The torque required to spin the engine might only be 100 ft/lbs instead of the 350 the engine can make. Now, I apply a load to the engine. It takes 200 ft/lbs of torque to move the load (ok, so we should be talking horsepower, but we'll stick with torque for a minute). For the engine to generate 200 ft/lbs of torque, it needs more fuel. So to hold 2500 rpms, we increase the amount of fuel provided by pressing down on the gas pedal. If we don't do so, RPMs will decrease (and the amount of work being done will decrease). Now, if we do work that needs 350 ft/lbs of torque to be done at 2500 rpms, I need to apply more throttle. At this point, I'm at full throttle. The engine is generating its maximum torque (350 ft/lbs) and I'm giving it all I can. If I remove load from the engine, RPMs will increase. If I reduce throttle, RPMs will decrease. The point is that simply operating at peak torque RPM doesn't mean operating at greater efficiency. While there may be some relationship, it is far more dependent on load. Running at 2500 rpms in a low gear with a light load is not as efficient as running at 1500 rpms in a higher gear with the same load. And running at 2500 rpms in a high gear with a heavy load may not be as efficient as running at 3000 rpms in a lower gear.

The one pretty basic principle of engine efficiency, given equal loads on the engine, is that increased RPMs means increased fuel consumption (hence specific fuel consumption charts that show a gradual increase in fuel consumption as RPM increases, even accounting for the % increase in RPM).

I would strongly guess that the reasons are related to something that went unobserved. Possibilities:
1. Difference in Shell Fuel (winter grade at one and not at the other?)
2. Difference in barometric conditions (wind, temperature, humidity, etc.)
3. Difference in road grade (uphill or downhill? Oh, wait...you said Texas... )
4. Difference in operative efficiency. On the way there, you were in more traffic and had to vary speed more often. On the way back, you were able to hold a constant speed. Perhaps you were drafting a truck part of the way?
5. Difference in how full you filled the tank.
6. Something else?

But simply increasing engine RPM a few hundred clicks when in the meat of the operating range (say from 2150 to 2300) is not going to automatically increase your mileage. There's a hidden force at work here.

 

Were you returning the other day when the wind was out of the north at 40 mph? I was trying to play golf that day and had a hard time hitting 300 yards into that wind. Wind can have a tremendous affect on mileage....and on drive distances.

Plus, when heading up to Dallas, you're going uphill and from Dallas you're coming back down. Don't laugh, there was a politician a few years ago stumping in the Dakotas. If elected, he promised to build all future roads downhill to conserve energy/resources.

Also, the tank filling theory is a very valid one. These trucks are notoriously hard to fill with the sensitive pumps of today. I've read countless posts whereby owners have a hard time getting their tanks full. Quite possibly you though you were completely full on your way up but weren't, hence upon refilling the tank you got more in. Alternatively, when you refilled after returning to Houston, you may have though the tank was completely full when in fact it wasn't.

There is absolutely no way in the world you can get better mileage by turning higher RPM's (inherently uses more fuel even though for a shorter period of time) and driving faster (increases drag) unless there are extenuating circumstances. The only accurate way to gauge mileage is to test many times thus reducing the error factors.

 

You must have been traveling with the spin of the earth on the way down and against on the way back. Three or more planets aligning themselves could also be the reason.

 

I drive 60 miles back and forth to work each day. It is all interstae driving except for a mile or tow on each end. I'm always driving 72-78 MPH and I've seen mileage from 11.8 - 17.9. It has EVERYTHING to do with the speed you are going and more importantly headwind or tailwind.

 

It just seems weird that the mileage goes up significantly when the speed goes up. I know that other factors such as wind, elevation, tank filling, etc., are definitely the culprits, but it is hard to believe that there is that much of a difference.

Gotta love the laws of physics.

 

That's strange because I drive tons of highway miles and my mileage is within 0.5 mpg every time regardless of conditions....never better than 15.