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5w40 Oil and Flash= lower mpg

Barry Mckeel

New Member
Apr 16, 2016
21
7
Truck Year
2016
Went on a 1500 mile trip, mpg down 2mpg to 25.Around town 14.

Think largest cause is flash changes shift points mostly in 5th or 6th around town stop and go.

Anyone else see the's changes?
 

Brady

Well-Known Member
Jun 7, 2015
1,042
416
Truck Year
2015
I've not gotten the latest flash. Not sure I want to either. Winter mpgs for me are bad enough, a good 4mpg hit. I couldn't stomach dropping even more.

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Zigi

New Member
Mar 21, 2016
23
6
Truck Year
2016
Went on a 1500 mile trip, mpg down 2mpg to 25.Around town 14.

Think largest cause is flash changes shift points mostly in 5th or 6th around town stop and go.

Anyone else see the's changes?
I agree, it seems that every time I get a new flash upgrade my mileage goes down. On my 2015, in 10,000 km I was averaging better than 10.5 liters per 100km.
My 2016 has 15,000 km and I'm averaging 12.5
My 2013 gas dropped a couple liters per 100 km after a flash as well
 

R Stanley

Member
Dec 22, 2016
41
23
Truck Year
2016
I got the ECM updates and TCM updates and oil update and running T6 oil now and my average has not changed, 25 to 27 on 2 lane roads driving 55 to 60ish, not a lot of stop and go, usually stops about every 8 to 10 miles depending on how far between towns. In town driving getting 21/23. Free way driving between 70 and 73, getting 26 to 27, 28, depends on traffic and elevation of road. . Have a 2016, 3.55 4 door, with air ride susp. Living in Arkansas, do not know if the weather or driving the type of roads Arkansas has laid out is the reason for my milage, but have seen milage very a lot between trucks when I read on posts and talking to people when on trips.
 

Huntinrod

New Member
Aug 7, 2016
16
7
Truck Year
2016
Regarding mpg: i bought a GDE tuned ECM. But i wanted to get my free first oil change. So i went to the dealer and got my free oil change. They needed to flash my stock ecm for the new Rotella T6 oil....no problem...the ecm was only gonna be in there another two hours! So they flashed my ecm. They also flashed my TCM (transmission). I thought...no biggie...go for it. Got it home and installed the GDE. I have seen no mpg improvement from the GDE....? Explain that. Some say winter fuel....i live in SoCal...not sure i get winter fuel here. Some blame cold temps...same thing...not cold here. All i can figure is the TCM....thoughts?

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R Stanley

Member
Dec 22, 2016
41
23
Truck Year
2016
Have you called GDE and talked to them about your fuel milage after putting it in.
 

GearHead

Active Member
Sep 13, 2016
284
98
Truck Year
2014
I have waited to post on the oil flash results until I had a consistent mileage history, well I think I have sufficient history with me and my wife driving. My average has settled out at 22mpg. My observation is, considering new tires, colder weather, we both have noticed that the shift points have been consistently 500 to 600 rpm higher than before, 3K vs. 2.5K. Transmission holds 6th and 7th for what seems like forever. Yes on the County back roads in OK with 50 mph speed limits I hate that it now holds 7th gear, I have to pull out of the throttle to get transmission to upshift and then gently roll back into throttle to hold 50mph. It also holds 7th up to 65mph on Expressway traffic, same tactic to get 8th to shift. Never had an issue before flash. Still happy with 22mpg average vs. my friends 2014 GMC averages 16mpg.
 

BoostN

Administrator
Staff member
Administrator
Jul 27, 2013
3,938
1,025
Truck Year
Not Listed
I have waited to post on the oil flash results until I had a consistent mileage history, well I think I have sufficient history with me and my wife driving. My average has settled out at 22mpg. My observation is, considering new tires, colder weather, we both have noticed that the shift points have been consistently 500 to 600 rpm higher than before, 3K vs. 2.5K. Transmission holds 6th and 7th for what seems like forever. Yes on the County back roads in OK with 50 mph speed limits I hate that it now holds 7th gear, I have to pull out of the throttle to get transmission to upshift and then gently roll back into throttle to hold 50mph. It also holds 7th up to 65mph on Expressway traffic, same tactic to get 8th to shift. Never had an issue before flash. Still happy with 22mpg average vs. my friends 2014 GMC averages 16mpg.

Aftermarket tuning could fix your shift points, but you run the risk of voiding a warranty..
 

R Stanley

Member
Dec 22, 2016
41
23
Truck Year
2016
Gearhead, I have not done this very often, always forget about except when towing, but have you tried manually shifting from 7th to 8th, or the next higher shift point you want instead of waiting to shift, especially on flat lands. Like I said have not done it very much, except towing, so not sure, just a thought.
 

cevans

Active Member
Nov 1, 2015
141
89
Truck Year
2014
I'm on 5w40 and did NOT get the flash and I am seeing lower MPGs. The 40 weight oil is the killer here, thicker at operating temperature means harder to pump = MPG loss from pumping losses.

The move to thinner oils (5w30, 0w30, now 0w20) is ALL about fuel efficiency. Thinner oils help manufacturers get that extra MPG or too they need for the car to make the target or CAFE standard. 40 weight oil WILL have an impact on this, thus my thread about class-action lawsuit.
 

GearHead

Active Member
Sep 13, 2016
284
98
Truck Year
2014
Gearhead, I have not done this very often, always forget about except when towing, but have you tried manually shifting from 7th to 8th, or the next higher shift point you want instead of waiting to shift, especially on flat lands. Like I said have not done it very much, except towing, so not sure, just a thought.
Actually I did and with mixed results. The DIC shows a request for 8th while still in 7th for about 30 seconds before an actual up-shift, but if any additional pressure is applied to throttle, transmission down-shifts to 7th. So little help. According to GDE their tune does not affect the TCM (Transmission Control Module) just the ECM (Engine Control Module).
 

Brady

Well-Known Member
Jun 7, 2015
1,042
416
Truck Year
2015
Are we sure that every truck has an updated flash concerning the change in oil weight?

My truck, built in Jan 2015, hasn't been to the dealer since I drove it off the lot in June of that year until last week. Couple of small issues I wanted to get fixed before the 36k mile mark so I took it in. (Oddly enough neither issue was resolved). The dealership said ECM & TCM were both 'up to date' and no flashes necessary. Are they full of it?

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