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

TXGREEK

By TXGREEK
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TXGREEK Senior Enthusiast

TXGREEK

Posted
Posted

 

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

7c850d64b42423ec711b3815bc552021.jpg

 

65 mph

dafb40d32e46bae69cbcb2abbedcd529.jpg

 

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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Mike GMC
Mike GMC

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

muddkatt
muddkatt

Run the same route again, but this time run 70 in the direction you ran 65 and run 65 in the direction you ran 70.

TXGREEK
TXGREEK

Ok, I believe I’ve figured it out, it’s definitely the wind cause today I was heading beck on the same road and low and behold This was yesterday coming back and this is today coming back ITS

Mike GMC Enthusiast

Mike GMC

Posted
Posted

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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TXGREEK Senior Enthusiast

TXGREEK

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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

 

Mike GMC Enthusiast

Mike GMC

Posted
Posted (edited)

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

Edited by Mike GMC
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CamGTP Senior Enthusiast

CamGTP

Posted
Posted

Doing a 2nd trip is what I'd do. Just to rule out anything weird. If tomorrow is similar to today's weather and you had time, I'd do it over again seeing how it wasn't a super long drive.

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TXGREEK Senior Enthusiast

TXGREEK

Posted
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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.

 

 

Sent from Above

 

 

Snowcamo Senior Enthusiast

Snowcamo

Posted
Posted
 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.

Sent from my LG-G710 using Tapatalk

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flyingfool Senior Enthusiast

flyingfool

Posted
Posted (edited)

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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diyer2 Senior Enthusiast

diyer2

Posted
Posted

Did you fill up with gas before your return?

Difference in gas?

 

:)

Z45 Senior Enthusiast

Z45

Posted
Posted

The faster you go the more aerodynamic drag (wind resistance) on the vehicle.  

TXGREEK Senior Enthusiast

TXGREEK

Posted
  • Author
Posted
Did you fill up with gas before your return?
Difference in gas?
 
[emoji4]


No, filled up before I left.


Sent from Above
TXGREEK Senior Enthusiast

TXGREEK

Posted
  • Author
Posted

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.


Sent from Above

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swathdiver Senior Enthusiast

swathdiver

Posted
Posted
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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garagerog Enthusiast

garagerog

Posted
Posted

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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huntr1117 Enthusiast

huntr1117

Posted
Posted

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.

Sent from my SM-G975U using Tapatalk

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