NoC

My cluster Mpg gauge is saying 17.4 mpg but my scan gauge I'd stating I am only getting 14.6 mpg. Anybody test the mpg?

KPSquared

The factory lie-o-meters are a joke. The only mileage that counts is what you've calculated by hand. Don't even bother looking at the displays. . .miles driven/gallons used. . .it's not rocket science and it's the only way to be accurate.

kik

Yep. You have to do the math yourself.

johnrb3b

NO!

Assuming your speedometer/odometer is calibrated correctly to your tire size and axle ratio, then you must log the miles from your odometer and the gallons you put in the tank each time. Then do the math.
You will find that the MPG computer in the dash is wildly optimistic - ie. it lies - way to the high side. It'll tell you you're getting 3 to 6 MPG better than reality.

So it goes.

MrDillClinton

Mine reads 22-23 mpg. Calculated, I get 19.

If you don't know how to do this (because some don't), here is what you do.

Run your tank until its almost empty...like 15 miles after your fuel light comes on.

Drive up to the gas pump and reset your Trip mileage counter to 0.

Fill up.... you have to fill all the way up or this doesn't work.

Next time you get gas, calculate the Trip Mileage divided by the number of gallons it takes to fill up your tank again. This will give you your true MPG.

Reset your Trip Mileage counter and do it again.

I do this every time I fill up.

MurderedOutRubicon

Im having the same issue with my 2012 Im going to calculate by hand

Freewill

Most gas stations are not level because they must drain when it rains.

When I fuel up at the same pump at the same gas station, my hand calculated MPG comes out exactly the same as the MPG computer.

If I fill up at different locations where the Jeep sits at a slightly different angle, my hand calculated MPG is usually different from the computer. The reason is simple: The Jeep only needs to be tipped only the smallest amount for the amount of air left in the tank to vary by as much as a gallon. This causes a big error in hand calculated MPG.

Unless you use a neighborhood gas station and you always park on the same side of the same pump, your hand calculated MPG will be the one with the error. The MPG computer does not have this problem.

Yeh, my JK is 2011, not 2012, but the MPG computer works the same way on both.

awesomo

Just like statistics, there are many different ways to calculate a metric. You just have to know how its calculated....unfortunately, I don't know how the Mopar ECU calculates the cluster MPG.

Different ways of calculating MPG (not industry standard; just logical mathematical calculations):

1) Miles Traveled/Fuel consumed [This is generally the most used and common calculation]

2) Miles Traveled/Fuel consumed-while not in idle [This calculation discounts any fuel consumed when the Jeep isn't traveling]

3) Average of sampled MPG at different intervals [The ECU will collect a single instant of MPG consumption at different points while moving and possibly while not moving, and then calculate the average of those samples]

4) Historical + Current MPG [Historical or past MPG rates are baked into your current MPG to calculate a life average]

There are many other logical ways to calculate MPG. I personally like to use method 1, since it accounts for stop-and-go traffic and street driving. But I'm gonna guess that many manufactures may not use this heads-up calculation; instead, they may use some other method (possibly one not mentioned here) to display a more efficient MPG reading.

To answer your question: Is the MPG reading on the gauge cluster correct? Yes/Probably, based on the assumptions and logic built into its calculation. But, I don't think the MPG cluster reading is calculated based on overall miles traveled divided by overall fuel consumed. If I were to guess, there are some other factors included in the calculation, explaining why most people's manual calculation never matches their gauge cluster reading.

LakeTravisJK

" I don't think the MPG cluster reading is calculated based on overall miles traveled divided by overall fuel consumed. If I were to guess, there are some other factors included in the calculation, explaining why most people's manual calculation never matches their gauge cluster reading. " ...
Exactly. The computer does not know precisely how much fuel was burned. Also, if tires have been changed(different circumference) and the speedo software was not adjusted accordingly, the speedometer/odometer will not be correct. In manual calculations, you can take this into account(adjustment after comparing to GPS measured true speed).

MrDillClinton

My dash reads 19.7 and I just calculated when I filled up at lunch: 18.9.

That's close enough for me. Hell, with all the idle I do in the cold weather to keep from running on a cold start, I'm definitely content with that.