I have seen a couple questions posted about mpg when using a tuner and I had the same question when I bought mine. I finally had the opportunity to test economy, not that it matters a whole heck of a lot. I'll state what I found right now, and provide some details if interested.

First off, I found I did get better mpg in the stock tune over the DP 80E tune. About 1.5 mpg better actually. I guess I was hoping the 80E tune would do better. Then again, it probably would if I drove extremely easy. But if I drove like that, why the hell do I need a program to increase power?

I found that driving without any tunes is boring, and I won't do it again unless I have too. Even though the mileage isn't as good, there were still times it was very noticable I was on the stock program and was wanting more. I'm jaded I suppose. So it's a pay to play kind of thing I guess.

I drove from the east coast of Virginia to Columbus, OH. The first tank on the way there revealed right at 15.8 mpg on 80E, I was running 70-74. Not the best really, but my truck is a CC LB 4x4. The drive starts out flat, then is very mountainous for a while, then flat again after you cross the OH river. I maybe could have done better if I took it easy on those steep up grades. But I just let it spool up and fly.

On the way home in stock form, I hit about 17.2. I don't think this is very good either, seeing as my dad said he got 19 out of his late 99. But he drove from IL and it was all flat.

It is possible the mpg are a little better than I calculated since I'm running 285s and they are taller than the stockers. My speedo is off 1-2 mph at 70 (it reads slow).

There's my take on it. The stock tune is the best for mpg, but IMO that's only because of it's limits. I could probably get the same out of my DP tunes, but I never will because the power is available and if it's there I'm going to use it.

On a side note, I love having the 38 gallon tank. I had no ambitions of doing that tank mod, but I'm pretty sure that's next on my list now. I'd love to be able to put 2-3 extra gallons in there.

 

You can put the extra fuel in there -- it just takes 10 minutes. LOL

And how did you make sure you knew *exactly* how much fuel was in there for your calculations? That's one of the reasons the tank mods are so attractive. When it's full to the neck, you know exactly how much you used. I got close to 20 MPG driving mine in the TW Daily-Drive tune like Grandpa, but like you said, it was no fun...

 

Ya, I don't know if there's something wrong with the truck, or me, or if other variables such as getting foam in the tank when filling it makes a significant difference in calculations. But I can tell you I have yet to get anywhere near 20 mpg in my truck.

It is pretty cool to be able to drive almost 600 miles from Columbus to my house without having to stop for a fill up! There was 1 pee break only on ride home.

 

Regarding your tire size, Ive found that the taller tires actually help my mpg compared to the stock size. Thats highway driving though, in town I suspect it hurts economy a little, but havent actually tested my economy out just in town.

Last year I drove 5 hours to get to work, and 5 hours back, every week so I got to get some accurate measurements.

I hear ya on the tank mod too, it always took me forever to fill the truck up to the brim to get accurate readings!

 

Going over 70mph puts you over 2000 rpm... bye bye mileage. When I still had my DP, I was running 285/75R16 Uniroyal Liberators @ 56 PSI cold, doing 70MPH from Denver, CO to Baltimore, MD I averaged 19.2MPG for the round trip and was sitting right at 2000 RPMs. Going over 2000 will hurt your mileage. Letting it "spool up & fly" up the hills... you shot yourself on that one. I think part of the reason your mileage suffered is that you played more, simple as that.

 

Most likely you are correct. I wasn't exactly trying to see how good of mileage I could get, but to see if there was a difference between a tune and no tune.

 

I think the difference in mileage is more due to your driving than the tuning. If you're going for a comparison in mileage capabilities, you'd really need to do the same trip exactly with the the only change being the tune. Same direction, fuel up at the same stations, etc.

Going over 70mph in these trucks means you'll NEVER get great mileage unless you've got a massive tailwind and are going downhill.

If you don't mind burning the $$$ to fill the tank, who cares, but the way you drove... the play factor is killing your mileage, not the tuning.

I drive my truck about 30,000miles/year so I know what playing does, and what driving @ 65 on cruise control does. I think if you did the same trip, set the cruise @ 65 and didn't rip up the hills, you'd see a difference.

 

Other than the direction, everything was the same. It's not like one way was uphill and the other way was downhill.

Maybe you misunderstand the way I drove. Most of the time it was with the cruise on. I wasn't accelerating up hills, I was maintaining. I was taking it easy for the most part; just driving, not playing. The majority of miles were at 70-71 mph. I get what you are saying, and if you reread my original post, the intent isn't to compare I get xxmpg, what do you get? It is to see if there was a difference in running a tune vs. no tune.