Have I been doing it wrong all these years?

If you're only going to keep the truck a couple years without a lot of miles then what odds. OTOH if you are going to keep it long term you can save a few bucks on fuel but eventually spend a lot of bucks on transmission. IMO at the very least follow the severe duty service schedule for AT fluid and filthers.

Terryallan wrote:

fj12ryder wrote:
The main thing to remember is that the MPG reading is a lie. If you really want to know your MPG, keep track of mileage and amounts added to the tank, over several tankfuls you'll get your pretty accurate MPG.

Let me understand. You trust your computer to tell you the number of miles you traveled. You trust your computer to tell you how fast you are going. You trust your computer to tell you the amount of time you traveled. You trust your computer to tell the engine what your foot wants it to do. You trust your computer to mix the correct amount of fuel with air, to allow the engine to run correctly.
But you don't trust your computer to be able to use all that information to figure your MPG? Instead you trust a hand held computer, and a simi accurate computer on a random gas pump on a simi level concrete pad to figure your MPG. Oke Doke!

But...But...But... I read it on the internet it must be true!!!

You guys are looking fir excuses to argue about stuff now.....sheesh.

To the OP, that seems like more of an anomaly than a phenomenon.
Mostly because 15mpg pulling a 30' 5th wheel doesn't seem very realistic.

N-Trouble wrote:

Terryallan wrote:

fj12ryder wrote:
The main thing to remember is that the MPG reading is a lie. If you really want to know your MPG, keep track of mileage and amounts added to the tank, over several tankfuls you'll get your pretty accurate MPG.

Let me understand. You trust your computer to tell you the number of miles you traveled. You trust your computer to tell you how fast you are going. You trust your computer to tell you the amount of time you traveled. You trust your computer to tell the engine what your foot wants it to do. You trust your computer to mix the correct amount of fuel with air, to allow the engine to run correctly.
But you don't trust your computer to be able to use all that information to figure your MPG? Instead you trust a hand held computer, and a simi accurate computer on a random gas pump on a simi level concrete pad to figure your MPG. Oke Doke!

Ok so on the flip-side if we assume my truck MPG readout is accurate than your implying every tested and certified gas pump Ive filled at is off by the same amount???

My truck always reads 6-7% above hand calculated numbers. This is directly attributed to “fuel used” value the computer tries to calculate. It is always 6-7% less than what the pumps say. These trucks have no gauge/meter to physically messure how much fuel the truck has actually burnt. Its all “calculated” numbers based on MANY engine parameters. A best guess estimate...

Some trucks are near spot on while others can be greatly off. Whichever you have will likey be consistent over the life of the truck.

Now back to the OP we all know nobody truly gets 15MPG towing a 30ft 5er out WEST. So just another datapoint that shows his computer numbers are BS.

Just saying. You use the computer generated miles to do your calculations. And you use a random gas pump, that may or may not have a level pad for the vehicle to sit on and if the one you used for the first fill did not have a level pad, or if the pad the vehicle sits on leaned slightly the opposite way. You are not getting a accurate reading.

For me. The computer is close enough. It has all the information needed to calculate MPG to a acceptable level.

TH mode generally holds the transmission in a lower gear longer and usually locks out top gear entirely. Both of these would cause a drop in mileage.

I think you will find TH mode most useful towing into a strong headwind or in the mountains, but on flat ground not so much. The reasons for wanting the transmission in a lower gear are to get the engine rpm up so the water pump moves more water for cooling under load and to get higher up on the torque curve for dragging heavy stuff up inclines, as well as improving engine braking going down.

On the flat, unless you hear the transmission "hunting," that is, shifting up and down a gear every few seconds, leaving TH off will save fuel. If the transmission hunts, though, better to engage TH mode.

I find TH mode cause the tranny to down shift sooner than needed and hold the gear longer than needed. I turn off TH when cruising for long stretches. I only use it in the mtns.

If the speedometer/odometer is off, which they usually are, even hand calculated numbers will be inaccurate. Manufacturers aren’t going to extend your warranty by an inch so it’s a pretty good guess which way it’ll be off. Our first Cummins Ram was 8% off. Taller tires brought it closer to reality. I tip my hat to the GM Dmax so no negative intended. I believe other circumstances contributed to the increased mileage.

Terryallan wrote:

fj12ryder wrote:
The main thing to remember is that the MPG reading is a lie. If you really want to know your MPG, keep track of mileage and amounts added to the tank, over several tankfuls you'll get your pretty accurate MPG.

Let me understand. You trust your computer to tell you the number of miles you traveled. You trust your computer to tell you how fast you are going. You trust your computer to tell you the amount of time you traveled. You trust your computer to tell the engine what your foot wants it to do. You trust your computer to mix the correct amount of fuel with air, to allow the engine to run correctly.
But you don't trust your computer to be able to use all that information to figure your MPG? Instead you trust a hand held computer, and a simi accurate computer on a random gas pump on a simi level concrete pad to figure your MPG. Oke Doke!

Ok so on the flip-side if we assume my truck MPG readout is accurate than your implying every tested and certified gas pump Ive filled at is off by the same amount???

My truck always reads 6-7% above hand calculated numbers. This is directly attributed to “fuel used” value the computer tries to calculate. It is always 6-7% less than what the pumps say. These trucks have no gauge/meter to physically messure how much fuel the truck has actually burnt. Its all “calculated” numbers based on MANY engine parameters. A best guess estimate...

Some trucks are near spot on while others can be greatly off. Whichever you have will likey be consistent over the life of the truck.

Now back to the OP we all know nobody truly gets 15MPG towing a 30ft 5er out WEST. So just another datapoint that shows his computer numbers are BS.

rollexx wrote:
I have been towing our 30ft fiver for years with a Silverado 2500 Duramax. Have been using the tow mode and get right around 12MPG. Today we headed out. I zeroed out the trip computer before leaving. Stopped at a rest area about 100 miles out, all freeway traveling at about 60 as usual. It’s then I noticed I had forgot to put it in tow mode. Checked MPG reading and was surprised to see 15 MPG. Tran temp and engine temp were normal so just left it off the rest of trip, another 140 miles. Checked again at end and got 14.4 MPG. So my question.. On fairly flat highways should I leave towing mode off and only turn it on when I hit the hills?

I have noticed that the TH mode often holds a lower gear MUCH longer than is needed. Using more fuel than necessary. I have turned off TH just to get it to change up.

fj12ryder wrote:
The main thing to remember is that the MPG reading is a lie. If you really want to know your MPG, keep track of mileage and amounts added to the tank, over several tankfuls you'll get your pretty accurate MPG.

Let me understand. You trust your computer to tell you the number of miles you traveled. You trust your computer to tell you how fast you are going. You trust your computer to tell you the amount of time you traveled. You trust your computer to tell the engine what your foot wants it to do. You trust your computer to mix the correct amount of fuel with air, to allow the engine to run correctly.
But you don't trust your computer to be able to use all that information to figure your MPG? Instead you trust a hand held computer, and a simi accurate computer on a random gas pump on a simi level concrete pad to figure your MPG. Oke Doke!