Correct Gas Milage - Computer Dash Calculations OR Hand Calculations??

[Maximus Gladius]

Which one offers the “correct” gas mileage numbers?

I have the 21 JTR, 3.6, 114k kms/71k miles.
Most people, if not all, have posted screen shots of their gas/fuel mileage and some are happy while others …not happy and I’ve always relied on the dash calculations to tell me what my gas mileage is doing.

Maybe it’s been said a couple times on here that the most accurate way of knowing is to hand calculate. Start with a full tank, or first shut off at the pump. Then drive and fill up again to first shut off at the pump. [ miles/kms divided by gallons/litres = mpg/…L/100 kms.]

So, in my 114k kms, I’ve only been watching the dash calculations… until yesterday. I filled up, had a short highway trip out of Calgary for the day then back last night and filled up again and hand calculated.

During the 113.7 km trip, my dash was showing me 14.5 to finish out at 13.6L/100km (17 mpg). The trip there was mostly strong headwinds and coming back it was relatively calm.

I stopped at the gas station and filled up again and came home and sat down to figure it out.

I drove 113.7 kms/13.3L = 8.5L/100 or 27.6 mpg! My dash isn’t even close. Now the real test is to drive at least half a tank, in the winter I don’t want to go lower than that. So, what have I done maintenance wise, mod wise to get to these numbers?

1. I drive the same brick you all do.
2. Mods like rock sliders and 315/70/17 tires - dealership calibration done. GPS matches odometer. Mods and tools I cart around = 250 lbs.
3. I do use an octane boost (technically it doesn’t change the octane # like choosing different grades of gas at the pump, it’s a “power” boost and doesn’t contain the bad stuff our book says to avoid)
4. I replaced spark plugs back at 60k kms with OE.
5. I installed an oil catch can and capture 210-250mls oil at every 5000 km oil change. I’m no longer burning this oil.
6. Swapped out oxygen sensors for OE at 110k kms.
7. Changed out the ERG valve and recalibrated it a couple weeks back.
7. Performed an ECM relearn, throttle body relearn, transmission quick learn.

That’s pretty much it.

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[Supazuk]

I don't look

 

[Janster]

I did a manual calculation one time (gasser) ….just to see the difference between the two. I filled up….. reset the MPG and trip info. Ran the entire tank, and did the calculations the next time I filled up again (Same gas station, same pump, yadda yadda yadda). Just normal everyday driving…..

And honestly? They were both the same….give or take a tenth or so. I was happy with the result and didn’t bother looking any further.

Are you saying…… 113.7 Kilomters (70.65 miles) and you used up 13.3 litres (3.513 gallons)? I’m coming up with 20mpg

Is your dash and/or computer setup for MPG or KPL?? Those numbers…14.5 / 13.6 is something I see on a regular basis in my gasser (Gallons/miles).

 

[mrhemi]

I have hand calculated mine many times for my '22 Gladiator. They have always been surprisingly equal. Any previous vehicle I have owned, that was not the case.

 

[starrskream]

Fuelly app work pretty well

 

[Freems]

I kept a manual log of fill ups for over a year on my 3.6, recorded also at the time of fill up what the dash numbers said and manual calculation…my numbers were very close to each other.

 

[Overland_Dreamrz]

If I saw a huge difference, I would have to suspect a speedo calibration issue.

 

[Mr._Bill]

A picture of your Fuel Economy display would be helpful.

The problem appears to be your math.

Your distance is in kilometers, and your fuel is in liters. Your initial calculation would be kilometers (113.7) driven per liters (13.33) used (8.53km per L). I don't know how you got 27.6 MPG from that.

I just did a conversion and the math to show MPG, which is similar to what I get on road trips.
70.65 miles (113.7km) divided by 3.52 gallons (13.33L) equals 20.07 MPG.

The fuel economy display on the truck is a running total of the fuel burned and the distance traveled. Any time spent idling (remote starts, sleeping with the engine running) will distort the numbers showing it to be lower than actual hand-calculated MPG.

I reset the Trip B meter every time I fill the tank. I never reset the Trip A meter (it auto resets at 10k miles). The Trip A meter and running total are real close to the running total kept by the fuel app I enter the data in. The Trip B meter is used to get the MPG per trip. I get between 15 and 21 MPG, depending on the type of travel and distance. On long trips, staying close to the Speed Limit, I usually get 21 MPG. I have a 2023 High Altitude with the factory 20 inch wheels and street tires. I got similar results from the 2020 Overland with the factory 18 inch wheels and street tires.

 

[Labswine]

I have found the dash vs. calculated to pretty much agree within +/- 0.3 MPGs. I get around 19 in the winter (winter blend gas) and around 21 in the summer. Typical highway is around 25 MPGs, with the best ever observed 26.7 MPGs (dash said 27...). I have the Overland, stone stock (ordered 5/19, delivered 7/19).

 

[Maximus Gladius]

I used a conversion app to take the litres/100 kms, then converts to MPG.

 

[denverd1]

I don't look

This is the way

Have you changed the jeep? Ie. Bigger tires or added anything else hanging off of it?

Not the best commuter and grocery getter

 

[Badunit]

I used a conversion app to take the litres/100 kms, then converts to MPG.

8.5 is not correct. 13.33L/113.7km = 11.7L/100km not 8.5L/100km. It is an impressive 23mpg but the accuracy is suspect because of the short distance and small amount of fuel. Pumps don't shut off at the exact same fill level every time. A difference of just 1/2 liter would bring your fuel mileage down to 13.83L/100km = 19mpg. Still good but not 23 or 28.

For better accuracy you could try waiting 30 seconds after it clicks off then trying to fill it more. You will also get better accuracy if you do it over a complete tankful. For best accuracy you do it over multiple tankfuls, keeping track of the total mileage and total of all the fillups then dividing.

I find the tank-to-tank mpg calculations jump around a lot for no good reason other than the most likely cause of not filling to the exact same level each time, but they average out over time.

 

[Biff Happy]

I used a conversion app to take the litres/100 kms, then converts to MPG.

I think your issue is where did you get 8.5L/100?
It’s more like 11.77/100.
if you had 8.5L over the first 100km, that means the next 13.7 km you used 4.8L. (35.04L/100km!)

 

[Maximus Gladius]

The problem appears to be your math.

I’ve run the #’s through two different apps. If the math is wrong, then I’m ok with that but I think the apps are right.

 

[Badunit]

Garbage in = garbage out. While 8.5/100 is 27.7mpg, the 8.5 used in your calculation is not correct.

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