Looking for MPG help - Page 13 - Ram Rebel Forum
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post #121 of 129 Old 02-14-2017, 01:21 PM
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Originally Posted by crappieduster View Post
Mine has read 1.5 higher than hand calculated every tank since new.
Yours is broken. lol

The list of all of my awesome mods: US Army Veteran decal
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post #122 of 129 Old 02-18-2017, 02:08 AM
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I got curious and started digging into the Toyo tire to see if it could be a contributor to our less than expected fuel economy and I found on the toyo web site were they rate their tires for fuel economy and the Toyo Open Country AT2 was rated at a 3. Some were rated at 2, most 3 but one which was a highway tire was rated at a 4. That is a good indication that part of the issues we are seeing could be tire related. I was really looking for the weight of this tire. I may take a tire and rim and weight then just to see what they weight together.

https://www.toyotires.com/tire/patte...-terrain-tires
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post #123 of 129 Old 02-18-2017, 03:26 AM
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Here is another article from fourwheeler.com that found in their specific testing 15% reduction in fuel economy just due to the tread design. More aggressive equals less efficient.
Tires and Fuel Economy - Four Wheeler Magazine

Regarding weight:
I did find out the Toyo AT2 tires weight in at 56lbs.
Comparison the General Grabber AT 2 is 53lbs
And to throw in my old 2014 RAM Wrangler SRA tires, they were 40lbs. 16lbs per tire less.

I would say, that I have been trying to compare the Rebel MPG to my Laramie and just the tires alone is a big difference. I am starting to believe that maybe if we want to use these AT tires with more aggressive designs, we should accept the fact that they are heavier and will be less efficient.
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post #124 of 129 Old 02-18-2017, 08:34 PM
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Good find. The physics of rolling friction is very simple math, even though a lot of things come into play. It's basically the product of the rolling friction coefficient between the two surfaces, multiplied by the total load on the tire.

These are 5,600 lb trucks minimum, so each wheel has about 1400 lbs of constant load. That includes the weight of the tire itself. This is important because the weight of the tire really has an almost trivial impact. Moving from your Toyos to your old Wranglers would reduce the load by 16 lbs as you noted, which would be about a 1% decrease in load and rolling friction.

However, the big difference is the coefficient of rolling friction between the tire and pavement. That's caused mostly by deformation of both as you drive. The article covers the factors involved pretty well. Sidewall stiffness, tire inflation level, tread flexibility. Anything which detracts from the stiffness of the tire such as softer tread, or more gaps between the tread, increases the rolling resistance. This can have impact between tire models of as much as 60%.

For example the rolling resistance coefficient of these tires is:
Goodyear Integrity: 7.36
Uniroyal ATM SRTT: 9.30
Bridgestone Turanza LS-V: 10.80
Pirelli P5 Four Seasons: 11.33

(these are all passenger rated P225/60r16)
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post #125 of 129 Old 02-18-2017, 09:31 PM
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Any idea what the rolling resistance coefficient is for the Toyo Open Country AT2 and how they compare to other AT tires?

Do you believe this specific tire could be one of the main factors behind the less than expected fuel mileage? I have to admit, I have never given much thought to tires affecting MPG.
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post #126 of 129 Old 02-19-2017, 01:53 PM
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To your first question, no, you'd have to contact Toyo and see what they think. I got those particular numbers out of an old study report. But I wouldn't expect dramatic differences between tires designed for the same goals. I'd expect just small differences between ATs.

But also, notice that those are all different tire types I listed from that report. The Goodyear Integrity is a basic all-season designed for mileage, the Uniroyal is tour-rated kind of middle-of the road tire, the Bridgestone is designed for extremely smooth ride and low noise, and the Pirelli was a "comfort" tire.

So those four tires progress from stiffer to softer rides, which probably explains the increase in coefficients.

To the second question -

The Toya AT is a pretty stiff tire, but the tread is more aggressive than any touring tire made, which increases the rolling resistance. You'd certainly get better mileage from highway tires. LT highway tires have smoother tread, and smaller gaps, if any, between successive tread blocks. Here's a pic of Nitto Dura Grappler tread, for example. Of course, they don't look as cool as something like the ATs. The Dura Grappler is designed for high mileage.


One thing I do believe is that Ram had to run their mileage tests on stock trucks in the trim model, and all the Rebels are sold with the Toyos. So I don't think that the tire explains any real difference between the tested mileage showing on your sticker and what you actually get. (I might be wrong about the tests, it's just what I think is supposed to be done.)

But, what I do know for certain will effect mileage:

Driving style

Low tire pressure (1 psi on a car tire = .7 to 1.2 mpg loss in past testing) With our higher pressure, the effect is lessened a bit, but still around half that.

Driving steadiness (steady foot = less changes in speed = better mileage)

Speed over 50 (+5mph = -3% to -10% mpg)

Speed over 65 (air resistance is high enough at that range that MDS starts to kick off over 65, as much as 8% mpg loss)

Speed over 75 (air resistance starts forcing mileage down even faster)

This chart is of the mileage I got when I ran a long series of tests. The blue line with the marked dots are from monitoring the truck computer over a number of miles on relatively flat highway, to see the highway mileage. You can clearly see the large drop in mileage between 65 and 70, about 7.5 or 8% difference? That's where the MDS was tending to shift off, depending on minor grade changes and wind direction, and of course how steady I was. The thin green line is just a projection of what the mileage might have been if MDS had stayed on. And the purple line is where the mileage might be with MDS disabled. I never bothered to actually test that.
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post #127 of 129 Old 02-19-2017, 11:40 PM
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Thanks John for your input. I tracked my 2014 RAM fuel economy over the life of the truck to about 50k miles. It averaged 17.2 over this span. I broke the average mpg into 5k mile average segments and noticed that each segment actually increased in MPG right up to about the 25 to 30k segment then it somewhat stabilized. I'll try to find the data and post it here if you are interested in seeing it.
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post #128 of 129 Old 02-20-2017, 12:46 PM
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Did your 2014 have the 5.7L Hemi with MDS ?

Three other factors on mileage I didn't mention, but should be obvious:

Window use - open windows / sunroof cause extra air drag

Grade - even a slight uphill grade is going to reduce your mileage. Mileage of going up a grade and then down the opposite side will be lower than traveling the same distance on level ground.

Accessories - power drains from accessories can add up. A/C use requires extra power from the engine to drive the compressor. Likewise, the alternator requires more drive power to run blowers, defrosters, audio amps, and high-powered lights, along with air shock pumps or anything else that might be loading the alternator, putting a mechanical load on the engine. The math is pretty simple. The alternator can handle 180A. 180A x 12V = 2160W maximum, or about 3 HP. (1 HP = 746 W) But the actual load is at least double that, because of loss of efficiency in the alternator as load increases, and because of conversion loss. Probably closer to 8HP equivalent.

Steady load on the alternator with all the accessories turned off is probably at least 20A, just charging the battery and providing power to the computers, sensors, instruments, and ignition, would be my wild guess, I would expect that there's usually at least 1 HP load due to minimum engine operation.

BTW, keep in mind that with all those numbers above, rolling resistance contributes only a small amount to mileage reduction at speeds over 45. The rest is all due to air resistance (drag).

This diagram shows it a little, but the curve is for an average automobile that's a lot more aerodynamic than our trucks. So the aero curve for our trucks would be higher and steeper.


And you can sort of translate it to HP (and therefore mileage) using this kind of chart. Just like the chart above, this one is not exact for our truck. It's just an example.
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post #129 of 129 Old 02-26-2017, 12:21 AM
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It was a 5.7 with MDS. Not sure how much of an effect it has but one difference is the Rebel doesn't have shutters. Not sure why they don't but my 2014 did.
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