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Break-in time vs bad fuel economy on brand new truck


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11 hours ago, Yondu said:

Ok, ok!  Tonneau cover works if you install a enormous gurney flap :). I think in essence the tailgate (with no cover) is like a gurney flap.  

 

 

I've never tested it, but it has been proven to decrease mileage.  As Ron Burgundy said...it's science.  

A Gurney 'flap' is just that, a flap. That one inch rise at the trailing edge of a wing. The wing in the photo is set for a shorter track to increase downforce. At longer tracks with high banks and large radius turns the wing is set nearly flat extending the length of the horizontal surface. At the point the 'Gurney" inhibits the spill of air breaking the vacuum behind the truck reducing drag. Low drag = more speed for same power = less fuel for lower speed. 

 

A truck bed is a turbulent pocket of air ended with a sheet of "plywood", aka tailgate. Oddly in the wind tunnel this turbulent pocket of air becomes 'as if' it were nearly solid near the tail gate. A cleaner exit than one might expect but the front half  is a mess behind the cab. Without a Gurney at the tail of a bed cover the spill over at higher speeds isn't much better than an empty bed. At moderate speeds however where the air remains attached it is a different story. About 25 meters per second. 

 

Aero is a system. No one and I mean no one test as if it is that writes about it.  Cover no Gurney. Gurney no cover. Gate up. Gate down no Gurney. Half covers no Gurney. Serrated vs parallel edge. Job is always half done to tell a story that someone want's told a certain way. Point is you never get the WHOLE story. 

 

Here's some truth for you. If you knew what worked or didn't work and how it worked and under what conditions it works you wouldn't be interested because? Because no one wants to drive 55.9 mph or less true air speed. To drive faster and benefit the entire system needs to be addressed as in the race truck photo. 

 

How well do you believe the Semi box trailer 'trailer tail' would work on an open top trailer of half height of the cab? 

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8 hours ago, Grumpy Bear said:

A Gurney 'flap' is just that, a flap. That one inch rise at the trailing edge of a wing. The wing in the photo is set for a shorter track to increase downforce. At longer tracks with high banks and large radius turns the wing is set nearly flat extending the length of the horizontal surface. At the point the 'Gurney" inhibits the spill of air breaking the vacuum behind the truck reducing drag. Low drag = more speed for same power = less fuel for lower speed. 

 

A truck bed is a turbulent pocket of air ended with a sheet of "plywood", aka tailgate. Oddly in the wind tunnel this turbulent pocket of air becomes 'as if' it were nearly solid near the tail gate. A cleaner exit than one might expect but the front half  is a mess behind the cab. Without a Gurney at the tail of a bed cover the spill over at higher speeds isn't much better than an empty bed. At moderate speeds however where the air remains attached it is a different story. About 25 meters per second. 

 

Aero is a system. No one and I mean no one test as if it is that writes about it.  Cover no Gurney. Gurney no cover. Gate up. Gate down no Gurney. Half covers no Gurney. Serrated vs parallel edge. Job is always half done to tell a story that someone want's told a certain way. Point is you never get the WHOLE story. 

 

Here's some truth for you. If you knew what worked or didn't work and how it worked and under what conditions it works you wouldn't be interested because? Because no one wants to drive 55.9 mph or less true air speed. To drive faster and benefit the entire system needs to be addressed as in the race truck photo. 

 

How well do you believe the Semi box trailer 'trailer tail' would work on an open top trailer of half height of the cab? 

I've noticed from past threads you like to turn light hearted conversations into chest beating "I know more than you-athons" often with incorrect or irrelevant info. 

 

Tonneau covers hurt your mileage, end of story.  It's been proven time and time again, but I still have one for other reasons.  

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I've noticed from past threads you like to turn light hearted conversations into chest beating "I know more than you-athons" often with incorrect or irrelevant info. 
 
Tonneau covers hurt your mileage, end of story.  It's been proven time and time again, but I still have one for other reasons.  
I realize both of you are likely to argue this to the end, but either way there is no solid proof that with our trucks you gain or lose any fuel economy with a tonneau. Everything I can find with documented proof behind it says you will get better fuel economy with a tonneau but never enough to justify buying it for that reason alone. And the fuel savings only happen above 55mph (100km/h) range. Even at that it's such a small difference it's barely measurable. The only reason you would lose fuel economy would be because of the added weight which makes sense in stop and go situations. I have a tonneau for functionality and cosmetic reasons. Not for fuel economy.

Please leave the debate in another thread if you two want to hash it out as I'd like to leave others here some useful info and results from my personal experience.

Thanks [emoji16]

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I got mine for keeping things out of sight and for the most part dry. If I was remotely concerned about fuel mileage I probably would not have purchased a truck.

Oops and now I see I’m not even in the tonneau cover thread...sorry folks ?

Edited by SS502
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To the OP...I did all my add on things within a week of purchase so I do not know if the level, bigger tires and tonneau cover made much difference but I can tell you that it has been very consistent since I’ve been driving it which may sort of rule out a break-in period. 

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13 hours ago, Loafer987 said:

I realize both of you are likely to argue this to the end, but either way there is no solid proof that with our trucks you gain or lose any fuel economy with a tonneau. Everything I can find with documented proof behind it says you will get better fuel economy with a tonneau but never enough to justify buying it for that reason alone. And the fuel savings only happen above 55mph (100km/h) range. Even at that it's such a small difference it's barely measurable. The only reason you would lose fuel economy would be because of the added weight which makes sense in stop and go situations. I have a tonneau for functionality and cosmetic reasons. Not for fuel economy.

Please leave the debate in another thread if you two want to hash it out as I'd like to leave others here some useful info and results from my personal experience.

Thanks emoji16.png

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No sir, I'm no trouble maker!  I just feel the OPs pain.  

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On ‎12‎/‎24‎/‎2018 at 4:55 AM, Loafer987 said:

Hello all! I've been reading posts on this site for the past 2 months or so to learn as much as possible about my 2018 sierra 1500.  I'm having a horrible time getting fuel consumption under control and I'm extremely confident my "old man" driving should be netting me much better fuel economy than what I am getting. 

I have a 2018 gmc 1500 sle elevation edition w kodiak package. crew cab, reg box, 5.3l 4x4 6 speed.  At the dealer I added the leveling kit, oem running boards, removed the air dam and upsized the tires to 285/60r20 on the stock wheels.  I added a hard tri-fold tonneau cover recently as well. I've never owned a new vehicle before so all of this is a learning curve for me.  The dealer keeps reassuring me my mileage will come down in time after break-in etc but every tank is a tiny bit worse and it's getting a bit concerning. 

According to my cluster and GMC app I have averaged 16.1 l/100 km (14.6 mpg) since the truck was new.  Now according to my Drivvo app I am averaging around 18.0l/100km (13 mpg) which is also what my math comes out to when I calculate it myself after the past few fillups.  I realize I have a few "bad for economy" mods on the truck but really my questions are:

 

1. Having only 3500kms on the truck so far, is the lack of break-in really the problem?

2. Has anyone else noticed the fuel consumption display in the truck being off by this much? 

3. Should I look at a tuner/tune to update the computer with the proper tire size and try for a tune that's better on fuel? 

 

I dont plan to tow anything with this truck other than dirt bikes in the box in the summer time, and I realize it's a truck not a car etc etc, but I have read enough posts to know I'm using almost 40-50% more fuel than the average person on here so I really just want to know what could be the issue considering the truck is literally brand new...

I am realistically 50-50 highway vs city driving. 

Thanks in advance!

 

Chest beating is for monkeys.  Not even an argument worth breath. SO directly to the OP then......

 

1.) I have had this lifelong habit of tracking new vehicle mileage from day one. I have a better than average grasp of statistics. My observations on break in vs fuel economy are that it will continue to increase to 40,000 mile mark. (65,000 kilometers)  Remain fairly constant for a few decades and tail off just before it expires. The assumes some reasonable maintenance is preformed in a timely manor and driven as an adult. This correlates well with wear studies from several major US Universities for decades. MIT has a few nice papers as does the University of Utah. OR run you own study. If you disagree...fine by me. 

 

2.) My dash reads 8-9% better than actual lifetime 85,000 miles of calculated data. If you care to do a survey I believe you will find it quite common for GM trucks. Maybe that's useful, maybe not. 

 

3.) I didn't look up your tire size and compare it to factory equipment but any difference in roll out distance will affect the display. Most hand held tuners adjust in 1/4' increments. That can help but.... There is one, don't remember the name that uses GPS data you collect with a logger. There's always professional tuning via data logger and mail in such as Black Bear. You get to decide it's utility for you. Not matter how well corrected the base calculation and internal calibrations will not allow it to function accurately. Might I suggest a Liner Logic Scan Gauge II? Cheap and independent of on board factory display marketing illusions.  It uses the ECM and can be calibrated by your data. 

 

Some notes:

 

Your choice of truck is one of the heavier ones. That hurts.

The front air dam and lower engine compartment cover are there to 'smooth' the air over the rough contours of the trucks bottom. 

The 'level' if done as a front lift instead of a rear lower increases the 'effective' frontal area. That hurts.

Larger tires (heavier) increases break away  and acceleration inertia. That hurts. More in town that highway. 

A bed cover is as much about shapes as it is about covering the void which I detailed earlier. Believe as you wish. 

Running boards can be a wash aero wise but add weight. Depends on their shape and placement. If find mine useful in rocker panel protection coupled with mud flaps and worth any cost they might invite.

 

There are few things as an individual element that have huge impacts but like pennies making dollars they add up. Tires and bed caps area a HUGE exception.

 

The TCM is a learning style AI that does so constantly with it's most notable influence early on. Shift points and application tables will effect mileage.

 

Aero drag is mainly about boundary layer separation. It is speed sensitive and is defined by the calculation of the Reynolds Number. This number defines the point were flow moves from laminar to turbulent. What's my point? Physics determines most things, math makes them intelligible to humans and opinions are not part of either form of discipline. Take them with large doses of salt. 

 

Most people are not patient enough to collect and process their data, if collected at all, to any meaningful conclusions thus is takes a hammer blow in variation to be noticed by Joe Average. Think in terms of percentage and not in raw numbers. For example a 10% decrease is notable but is only 1 mpg on a 10 mpg base but 3 mpg on a 30 mpg base. It also means that opinions on forums are rarely based on anything actually meaningful but are more common that poop in a chicken coop.

 

Your results are just that, yours. Directionally things can be common but in the finite they are unique. Your truck in unique. Your driving and maintenance habits are unique. Your area, topography and weather and traffic is unique. Get your happy on and enjoy it.  

:seeya:

 

 

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On 12/24/2018 at 9:57 AM, Yondu said:

Your toneau cover is actually hurting mileage too.  It's a myth it increases mileage.  

I don't know if that is true. I agree that the tonneau cover does not improve mileage, but I don't think it hurts either. Kind of a negligible difference?

 

Anyway, to the OP, not a lot of talk about winter blend fuel vs summer blend.

 

My mileage drops drastically in the winter, not only due to extended warm up times, but just because the fuel blend is different. Winter blend is also cheaper so your mileage per $ may remain the same, but each tank lasts a few less miles. So be careful comparing your numbers in December to what anybody posts on the forum the rest of the year. I'm guessing all EPA testing for fuel economy occur in warm months with summer blend fuel. Now based on your metric numbers, you must be up in Canadia, so I don't have any clue about your local fuel, etc. Maybe someone else does.

 

My DIC reading is usually about .5 MPG higher than hand calc. I think the truck's calculation fails to calculate fuel wasted during remote starts, etc. I feel like the DIC calculated number is fairly accurate for while you are in motion, but since you also burn fuel while idling and warming up, the actual miles per tank are less.

 

All that said, your stated mileage is not that far off of expected for someone with a lift and large tires. I doubt your truck has any issues.

Edited by aseibel
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In off topic I wrote a post on fuel. 2% difference between winter and summer blends in BTU content. Source is stated in post. Not a major factor. The 54 F average temperature delta from summer to winter is. (at my latitude) Major effect on cold start/run power due to vast increases in lubricant viscosity and extended warm up times to same end point temperature. 

 

Hot August day in AZ and your start your car when it's 104 F out. Oil viscosity is 63 cSt

Cold Wisconsin Evening at -22F viscosity is 5840 cSt. A factor of over 90 X!

Both heat to 212 F and at that temperature the viscosity is 10.6 cSt regardless of outside temperatures. 

 

(Example Quaker State UD 5W30)

 

I've posted the extremes for which there is actual data. Easy to note the even on a hot day the oils viscosity is 6X minimum more viscous before and after warm up. Winter will double or triple that in most locations where is actually matters. 

 

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Try adding more air to the tires. If they are the LTs, they likely have an 80 psi max.  36 psi is less than half of the max pressure.  Let's say each tire has a 3750 pound load capacity at max pressure.  At 36 psi, they should hold about 45% of max or 1688 pounds.  That should be more than adequate, but you will get less rolling resistance  at a higher pressure.  I find 50 psi works well on my LTs on my older truck.  You might check to see what the factory pressure spec is on the 3/4 ton trucks with 20" wheels.  Since you are running a non stock size and load rating, it will be a bit of trial and error to balance mpg and proper wear patterns.

Edited by Mike GMC
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+1 on the tire pressure. Having 285/65/20 michelins able to go to 80psi I keep them at 45 and they roll incredibly well.

 

As far as mileage ...plain and simple every POUND or KILO for some of you, you add takes more fuel to accelerate. So if a Tonnaeu weighs 80 lbs it takes X more fuel to set it in motion from a stop. If the aero it adds at highway speed is not enough to make up the difference it is a net loss of mileage. My own observation has been that the shorter bed on my current 17 has much less Aero issue than my old long bed Ford did so my guess is a cover lowers mileage slightly. During summer I also drop my spare tire in the garage and get a small mileage bonus also.

 

To correct your mileage without tuning for tire size take the truck to highway speed ~75 and look at a GPS and note the ACTUAL speed and divide the two numbers GPS/DASH. Example: 79mph GPS / 75mph DASH = 1.053 (5.3%) and multiply your dash estimated mpg by that number for true MPG. ...this is BC with larger tires you are traveling farther than the dash is reporting.

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  • 2 weeks later...
+1 on the tire pressure. Having 285/65/20 michelins able to go to 80psi I keep them at 45 and they roll incredibly well.
 
As far as mileage ...plain and simple every POUND or KILO for some of you, you add takes more fuel to accelerate. So if a Tonnaeu weighs 80 lbs it takes X more fuel to set it in motion from a stop. If the aero it adds at highway speed is not enough to make up the difference it is a net loss of mileage. My own observation has been that the shorter bed on my current 17 has much less Aero issue than my old long bed Ford did so my guess is a cover lowers mileage slightly. During summer I also drop my spare tire in the garage and get a small mileage bonus also.
 
To correct your mileage without tuning for tire size take the truck to highway speed ~75 and look at a GPS and note the ACTUAL speed and divide the two numbers GPS/DASH. Example: 79mph GPS / 75mph DASH = 1.053 (5.3%) and multiply your dash estimated mpg by that number for true MPG. ...this is BC with larger tires you are traveling farther than the dash is reporting.
Yes I was aware of the difference in distance and I know I was out by about 5%. That said I have finally had the chance to try some other tires and I'm completely blown away by the results.

UPDATE:
My last tank I swapped the tires around the 60km mark on my trip display. (I reset the trip A display every fillup and completely fill every tank) I swapped to a set of slightly used studded winter champiro ice pro 2s tires) they are on 18" rims and they are less than 1% larger than stock if they had full tread. They have approx 70% tread left. New tires are also 35psi as they were mounted by the dealer and that's what our trucks are rated for stock.

Just on this tank since changing tires only (still no air dam) I have dropped my average economy to 14.5l/100 kms based on the cluster calculation. About 15.5 when doing actual calculation.

My driving habits I've tried as hard as possible not to change but I did have one more 60km "highway" trip than usual.
This puts me over 15mpg and I expect the next tank to be better again without the first few kms of oversized tires. It's still not as good as I'd like but way better than I was getting.



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  • 4 weeks later...
On 2019-01-07 at 4:22 PM, Loafer987 said:

Yes I was aware of the difference in distance and I know I was out by about 5%. That said I have finally had the chance to try some other tires and I'm completely blown away by the results.

UPDATE:
My last tank I swapped the tires around the 60km mark on my trip display. (I reset the trip A display every fillup and completely fill every tank) I swapped to a set of slightly used studded winter champiro ice pro 2s tires) they are on 18" rims and they are less than 1% larger than stock if they had full tread. They have approx 70% tread left. New tires are also 35psi as they were mounted by the dealer and that's what our trucks are rated for stock.

Just on this tank since changing tires only (still no air dam) I have dropped my average economy to 14.5l/100 kms based on the cluster calculation. About 15.5 when doing actual calculation.

My driving habits I've tried as hard as possible not to change but I did have one more 60km "highway" trip than usual.
This puts me over 15mpg and I expect the next tank to be better again without the first few kms of oversized tires. It's still not as good as I'd like but way better than I was getting.



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I read this thread with a lot of curiosity. Just got a 2018 Sierra 1590 5.3L, levelled it, added tonneau cover and swapped up to 20” rims with 275/65R20 Toyo Open Country CT’s (34”). I’m averaging 20.7 L/100KM’s so far. Brutal. I wouldn’t think it was so bad if I hadn’t traded up from a 2016 F150 5.0L, also levelled with tonneau on 20’s (285/60R20 Firestone Transforce). I was getting 16L/100KM average in it. The Toyos are bigger and far more aggressive, might be why it’s so much worse. 

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4 hours ago, GMCerpillar said:

I read this thread with a lot of curiosity. Just got a 2018 Sierra 1590 5.3L, levelled it, added tonneau cover and swapped up to 20” rims with 275/65R20 Toyo Open Country CT’s (34”). I’m averaging 20.7 L/100KM’s so far. Brutal. I wouldn’t think it was so bad if I hadn’t traded up from a 2016 F150 5.0L, also levelled with tonneau on 20’s (285/60R20 Firestone Transforce). I was getting 16L/100KM average in it. The Toyos are bigger and far more aggressive, might be why it’s so much worse. 

Noooo, it’s because you’re driving that special edition Sierra 1590 ? no seriously you have a slightly bigger engine, bigger more aggressive tires and that F-150 was probably completely broken in and runnin a tad loose vs new and tight. What gear was the Ford vs GMC?

Edited by SS502
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