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Huge difference, large mpg loss 65 mph to 70 mph. Any idea what may be causing it, my gears?


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Took a short 30 mile trip today, decided to test my mpg’s via trucks computer, ran 65 mph one way and 70 mph the next.

 

70 mph

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65 mph

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Could it be my gears?

 

2018 GMC Sierra 1500, SLT, SuperChips tuned 6.2, 323 rear end. Lifted 4.5-5” running 34” Nitto Ridge Grapplers. Only run super unleaded. Why would there be such a mpg difference from 5mph??

 

 

 

 

 

 

 

Sent from Above

 

 

 

 

 

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Wind direction, grade, engine temperature may all contribute, but it looks like you found the sweet spot of your set up.  We have a highway here that seems pretty flat, but going south, I get 16.x and going north I get 21+.  Same speed both direction with the cruise set.  It must not be as flat as it seems.

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Wind direction, grade, engine temperature may all contribute, but it looks like you found the sweet spot of your set up.  We have a highway here that seems pretty flat, but going south, I get 16.x and going north I get 21+.  Same speed both direction with the cruise set.  It must not be as flat as it seems.

 

It was pretty flat and took same road there and back. That’s a big difference don’t you think?

 

It was approx 8-9 mpg difference which nothing changed, no added weight, nothing. If anything maybe the wind but still wouldn’t have caused that much of a difference, would it?

 

 

 

Sent from Above

 

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Speed and wind seem to make big differences on my bone stock 18.   I get bad mileage normally because I like to go fast.

 

Me too, I know it’s a truck and I didn’t buy it for mpg’s and I usually drive 85 plus on road trips never ever looking at mpg. I was getting 25-26 mpg bone stock lol

 

Maybe the lift has a lot to do with it on the trip back, could be.

 

 

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 Me too, I know it’s a truck and I didn’t buy it for mpg’s and I usually drive 85 plus on road trips never ever looking at mpg. I was getting 25-26 mpg bone stock lol
 
Maybe the lift has a lot to do with it on the trip back, could be.
 
 
Sent from Above
 
 
Mines the same. So i do the grampa and keep it at 60mph. 65-70 and i watch the gas needle drop on long trips.

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wind drag become exponential past 55 mph, this is why when we had an oil embargo in the late 70's-80's top hwy speed was regulated to 55mph.  save fuel.   could also be issue with your aftermarket parts and ecu tune . 

also having a lift kit creates more drag.  usually a lift kit throws the alignment off and makes more drag on the wheel to road surface. so one direction on hwy will have less drag in regards to your tire alignment , then coming back it will act different , also , temp. humidity ... etc    

 

i would not use the computers numbers, fill tank up, and note milage this is way more accurate. do your own math

Edited by flyingfool
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Ok, had to go back for an install, this time with cruise set to 74, I got 20+ mpg’s driving back to the same location, crazy just how much 5-10 mph difference can affect the mpg’s.


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17 hours ago, TXGREEK said:

Took a short 30 mile trip today, decided to test my mpg’s via trucks computer, ran 65 mph one way and 70 mph the next.

I've run multiple two way averages in 5 MPH increments to create a log for expected MPGs at a given speed.  That being said, mine loses 2 MPG going from 65 to 70 MPH, big brick flying through the air you know.  Worked with BlackBear to see if we could tweak the numbers but she just started pinging, was already as good as it was going to get on 93 octane gasoline.

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Pick up trucks, especially lifted ones have a coefficient of drag about like a barn door. So as speed goes up, they get exponentially worse mileage than your more aerodynamic grocery getter sedan sees as a drop-off in mpg with increasing speed. Lower numerical rear end gearing also is a factor when bucking a head wind or fighting an uphill elevation change, the engine just has to work harder. Great when you have a steep downgrade and a stiff tailwind, not so much visa versa.

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Where are you at in Texas? I'm in the Houston area and on my way to work this morning the North wind was brutal. Gusting at 21 mph. It was the 1st time I've even noticed wind noise coming from my A-pillar lights and it was LOUD. Definitely made me reconsider mounting a light bar behind my grill and getting rid of the A-pillar lights. Driving into a N wind gusting 21 mph will make a huge difference in your mpgs.

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