I"m trying to calculate cost per mile. I forgot what mpg I used to get with 91. If anyone has recently tried both or remember trying both at resonably stable gas price fluctuation, can you post what your differences were?

If the difference is only 1mpg at $3/gal-87 and $3.25/gal-91 then the cost per mile is exactly the same.

If the difference is 2mpg at $3/gal-87 and $3.25/gal-91 then the cost per mile is $.01/mile cheaper with 91.

So, basically it is approximately a $.01/mile decrease with every mpg increase using 91 vs 87 (octane).

 

I live about 5 miles from my office, so I don't spend a lot of time cruising at a constant speed. Instead, much of my driving time is spent speeding up, stopping at stop signs/stop lights, idling, etc. Therefore, I can expect to get much lower gas mileage than someone with a longer, more constant commute. Recently I've been thinking about this in a little more detail. It seems as though on the occasions that I do take the truck on the highway and stay at a constant speed, level 3 seems to really pay off. However, during my normal driving (described above), it almost seems as though level 2 pays off. In other words, to me it sometimes seems like I get better gas mileage (or at least the same gas mileage) in level 2 when accelerating than I do in level 3 when accelerating, all other factors being equal. I know level 2 increases the low-end torque (which I'd be using more often when starting from stop signs/stop lights), so I wonder if level 2 makes more sense for "city-type" driving and level 3 makes more sense for "highway-type" driving.

Anyway, to answer your question, I typically get pretty close to the same fuel efficiency in either level. Not sure how much could be attributed to my driving environment and how much could be attributed to my driving style. I don't think I drive that much differently between the two levels, but maybe I do.

You're thinking about whether or not level 3 is economically valuable the right way. I created a pretty simple spreadsheet to help me determine whether or not I should use level 3. You can plug these formulas, the cost per gallon, and your MPG for each level into the cells listed, then let the spreadsheet tell you which level to use.

Cell A2 = cost per gallon of 87 octane
Cell A3 = MPG when on level 2
Cell B2 = cost per gallong of 91 octane
Cell B3 = MPG when on level 3
Cell C2 = "=(B2-A2)/A2" (without the quotes)
Cell C3 = "=(B3-A3)/A3" (without the quotes)
Cell A1 = "=IF(C2>C3,"Use Level 2","Use Level 3")" (without the outside quotes - you need the quotes inside the parenteses)

After you plug in these formulas, the price per gallon, and your average fuel economy on each level, just do whatever it says in cell A1.

 

Thanks for the spreadsheet. I didn't think of that.

Well, here is the problem though. I tested using the Evo Avg MPG.

17.1mpg with 87 16 miles @ 60mph level 2.
18.1mpg with 87 16 miles @ 60mph level 3 but -2 degress timing retard.
17.1mpg with 91 16 miles @ 60mph level 3.

That didn't make any sense to me untill I read your city driving. Since I have 37x13.5's I think the tow settings help with much needed torque.

 

On the canned tunes, I got about 11% better mileage on Level 3 over Level 2. That was enough to pay most of the difference in cost, and 3 was a lLOT more fun. Drove about 30,000 miles on it.

 

I tried again today but had to reset on freeway becuase of traffic so I got 18.1mpg with 91oct @60mph on level 3, which is what a 9% or 1mpg increase. But I need a min of 1.5mpg to justify the 91 oct pump price difference.

 

It "sounds" like you're relying on either the Edge Avg MPG readout or the truck's (if you have the "Information Center" like I do). Neither is particularly accurate and if you "reset" the Edge, it takes 100,000 samples (one per second) to actually reach a stable average (that's 27.8 hours, to put it in more familiar terms). I don't know how rapidly the truck's computations stabilize - they DO seem somewhat different.

So, the data you are using, if this is the case, is not really valid. However, it MAY be good enough to determine the relative differences between the two fuels. You really need to run about 3-4 tankfuls of each fuel and compute the average mpg of each over that test period to get a realistic estimation though. I've found the "display Avg MPG" is usually 0.5 to 1.0 mpg higher than what is really happening, determined through an old-fashioned, "hand" calculation.

- Jack

 

That's funny. My display center MPG is almost always 0.2 MPG lower than my hand calculated (or, actually, spreadsheet calculated - but I've already proven I'm a dork) MPG is. Like Jack pointed out, the Evo calculated MPG differently than the display center and apparently differently than my spreasheet does. My Evo display ranges anywhere from 1 to 3 MPG higher than my manual calculation.

 

We're actually pretty close in our observations I think (but I never would have thought of using a spreadsheet to calculate gas mileage)! Like you I find the "display/information center" (whatever the heck it's called), seems closer to actual than what I see on the Gryphon. There've been times when the center value and my hand calculation absolutely agree. Most of the time though, it's just a bit high (usually in the 0.5 mpg range). But, the Gryphon number is always somewhat higher - why? Dunno. Has to be the algorithm used in the calculation. I know what Bill uses in the Gryphon, but I don't know what the dash display uses. And, just so everyone knows, the calculation Bill uses is mathematically correct.

I suspect some of the problem is wrapped up in the time interval / number of samples used by the Gryphon. Nearly 28 hours of driving is a LONG average. It could easily be about six tankfuls of gas. Watching the dash display, I'm pretty sure it averages over a much shorter period, possibly around four hours, or about one tank of gas. On a trip, of course, I calculate the mileage by dividing the total distance driven by the total gas used. Otherwise, I just do it per tankful. I often get 2-3 mpg variation in the results doing it at each fillup.

- Jack

 

I was using the EVO's AMPG but I reset just before freeway entrance and used cruise control set at 60. Under 65 at a constant pace is the best possibly milage. Otherwise it is not acurate enough combining city driving as stops, traffic and other factors skew the results.

Using the spreadsheet I need a minimum of 1.5mpg increase using 91 over 87 to justice the pump price difference. So far I have only been able to get 1mpg. I don't want to wait six tank fulls to put my order in. Allthough, it is possible I could get 1.5mpg increase with a Gryphon tune using 91 which is why I posted to get other peoples results be fore my order is sent in.

Unless someone else with a lifted truck has got at least a 1.5mpg increase with a 91 performance tune, then 87 is my best bet. Also since I have not change my gears it is possible a tow tune may yeild better then a performance tune.

 

I had about a 0.5 mpg increase in mileage with a custom 87 octane tune compared to the canned 87 octane tune. My guess is, you'd see about the same improvement between a custom 91 tune and the canned one.

I kind of doubt a tow tune would do better than a non-tow tune. It's set somewhat richer to keep temps down and the shift pattern is set to get you moving from a stop. If you're looking for best economy, ask for a tune that does that!

- Jack