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Old 09-20-2018, 02:10 PM   #1
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Name: Bob Ruggles
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Gas mpg

Any opinions on how reliable the truck computer is in displaying mpg? On a 300 mile trip mine showed 23.4 mpg. I have a 2015 Ram 1500, 4x4, crew cab, 5.7 liter hemi with deactivation of 4 cylinders when conditions allow. I have a 3.21 axle. I’d like to believe the computer but I’m a chicken and don’t want to hand calculate for fear that I’ll be disappointed.
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Old 09-20-2018, 02:29 PM   #2
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Originally Posted by rgrugg View Post
Any opinions on how reliable the truck computer is in displaying mpg? On a 300 mile trip mine showed 23.4 mpg. I have a 2015 Ram 1500, 4x4, crew cab, 5.7 liter hemi with deactivation of 4 cylinders when conditions allow. I have a 3.21 axle. I’d like to believe the computer but I’m a chicken and don’t want to hand calculate for fear that I’ll be disappointed.
I have found, at least in Lexus vehicles, that the real time mpg is not spot on, but that the total mpg calculation over that of the whole tank is pretty much within a couple mpg of doing it with pen and paper. Towing a scamp 13 bunk version, 2 liter 4 cyl turbo, i averaged 19mpg on a 5,000 mile trip with lots of wind and mountains, hills in the mix.
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Old 09-20-2018, 04:16 PM   #3
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Better hand calc it.

That mileage number sounds very high for that truck and Rams are known for having "optimistic" mileage computers.

I'm sure you weren't towing, but were you going down wind the whole way with a speed of only 45 MPH, on level ground with the cruise control on?
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Old 09-20-2018, 04:24 PM   #4
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Stressing about mpg is bad for your health.
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Old 09-20-2018, 04:30 PM   #5
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Mostly level mostly 4 lane highway with cruise set at 60. Of the 300 miles probably only about 40-45 on two lane roads and no city driving.
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Old 09-20-2018, 04:32 PM   #6
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I don’t stress about it. Whatever the truck gets it’s better than the previous Chevy Silverado 1500. The Ram had an 8 speed transmission while the Chevy had a six speed and the Chevy had a higher numerical axle.
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Old 09-20-2018, 04:34 PM   #7
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Gas mpg

Accuracy varies. Mine shows 1-2 mpg higher than I calculate manually.

When I first got the vehicle (my first with that feature) I tracked it manually for several months and compared the result with the on-board computer. Manual calculations can have variation, too, as not all gas pumps shut off at the same full mark.

If you want to know your fuel mileage to improve your driving habits or to be aware of a sudden, unexplained change that might indicate a mechanical problem, great (and it really doesn't matter if it's spot on or not). If you're looking for bragging rights, get over it. I never believe what anyone else says about their fuel mileage, anyway. It's in same category as age and weight. [emoji6]

Have I mentioned I like your signature line, Raspy?
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Old 09-20-2018, 09:48 PM   #8
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They don't call it a lie-o-meter for nothing.
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Old 09-21-2018, 07:29 AM   #9
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Originally Posted by Jon in AZ View Post

Have I mentioned I like your signature line, Raspy?
Thanks. Yep, exaggerating in the interest of truth.
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Old 09-21-2018, 07:13 PM   #10
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My 2016 Tacoma is very accurate based on tank mileage. That is what the display shows. Its not showing actual mileage at any given moment. However it caculates it, its doing a good job.
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Old 09-21-2018, 08:23 PM   #11
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even measuring your gas fills and dividing by mileage can vary by several MPG per tank as not all gas station pumps will fill the exact same level in your tank, so I ge tthe most accurate readings when I average several tanks..
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Old 09-22-2018, 05:35 AM   #12
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All my Toyotas have had a speedometer that read high, and that is very common. Better to err on the high side than the low side they figure. And they seem to show a correlated error in odometer and MPG reading. I pay no attention to the MPG readout. Using the "tank-fill" method yields pretty consistent numbers but there will always be some variation...

https://www.fueleconomy.gov/feg/why_differ.shtml
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Old 09-22-2018, 05:51 AM   #13
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Is it not better to use the cars odomerter. That way you have the average for towing and not towing. After all that is the real cost of a trip. My rav, reads 20.5 for towing and not towing over a length of time. Just my thoughts. Carl
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Old 09-22-2018, 06:57 AM   #14
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It comes down to motive. Why do you want to know?

If you want to compare one vehicle to another, you're better off using a source like Consumer Reports that uses a consistent testing methodology with calibrated instruments.

If you want to monitor changes over time, just watch the on-board computer. It doesn't matter if it's precise or not- you're looking for change. It wouldn't hurt to verify manually occasionally in case the computer is the problem. I agree that averaging several tanks is best when computing manually.

And if you want to make accurate statements in public discussions, use any method you want. There are far too many variables to make the conversation helpful. I suspect most people take fuel mileage claims with a grain of salt and a shrug.
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Old 09-22-2018, 07:42 AM   #15
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Name: Bob Ruggles
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When I hear towing fuel mileage claims most of the time I don’t believe it. I have towing experience going all the way back to 1962 and have towed just about every kind of rv trailer so have a pretty solid idea of what mpg is reasonable.
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Old 09-22-2018, 10:12 AM   #16
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Originally Posted by gordon2 View Post
All my Toyotas have had a speedometer that read high, and that is very common.

How do you know this?
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Old 09-22-2018, 10:12 AM   #17
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Only one way to find out the computer's accuracy: FIND OUT! Fill the tank, record the odometer reading, drive, refill the tank, and calculate the MPG. My 1989 Lincoln Continental was always spot-on. My current 2004 Mercury Sable is always 2-3 MPG optimistic. I guess the technology was better back in the 80's.
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Old 09-22-2018, 10:44 AM   #18
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Life...

Life's too short to spend time worrying about MPGs.... while towing my hotel suite on wheels (with kitchen!) from place to place...whatever it costs...it costs.


I could always "go back in time" and travel by motorcycle with a pup tent strapped on the back of it... stellar MPG numbers there.....but that ain't happening!
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Old 09-22-2018, 11:00 AM   #19
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When I was a kid we’d calibrate the odometer using mileposts and assume that the speedometer error correlated. GPSs ended that.

The lie-o-meter on my ‘11 F-250 (anecdotally, I never recorded the numbers) was from .3 to .7 MPG optimistic.

My F-250 died a terrible death (with me in it) in a hailstorm. The lie-o-meter on my ‘17 F-350 that replaced it, based on only three fuel refills, is about .2 to .3 MPG optimistic. On both trucks I assumed that the odometer is correct since the speedometers seemed to have only minor errors.
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Old 09-22-2018, 11:08 AM   #20
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Name: Steve
Trailer: 2018, 21ft escape— 2019 Ram 1500 Laramie
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I use my onboard computer to occasionally check my MPG’s more out of curiosity than anything else. I never expected great fuel mileage when towing and have never been disappointed
When we are going on a trip , I am more concerned about getting there safely and comfortably then fuel mileage
If fuel mileage was my major concern then I would probably sell my trailer and stay home .

My vehicle speedometer matches my GPS speedometer exactly . Since I drive the speed limit and seldom faster than 62 MPH , that’s close enough for me
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