I drive a 2012 JKU manual on 35s. I average about ~17.5 mpg with speedometer matching my GPS. Mind you, I've tuned it with a FlashPaq for 87 performance mode and don't drive constantly over 70mph on highway. Hope this helps!

 

The only way to get your speedometer/Odometer reading correctly is to use GPs, run the car to 60MPH according to GPS and look at your speedometer. If your speedometer is actually showing less than 60, change the tire size one step larger (quarter inch) and do it again. Keep repeating this until your speedometer is totally agreeing with GPS. Now you can actually measure distance traveled in a tank of fuel because you found the actual multiplier for your Jeep computer to use that actually is a match to your tires. With my 37" STT Pro I had to use 35.25" to get it dead nuts on.... not 1 MPH off, but dead nuts on.

At 36.5" the numbers are off by a very large margin when trying to use the Lie-O-Meter to get MPG or miles traveled over a tank. Now that I am nailed in, my miles and gallons per tank (total miles divided by total gallons to refill) is always within a half MPG of the Lie-O-Meter. It's good enough that if I forget to reset after a fill up I know I can at least be close by looking at the dash.


Perform absolute calibration as described above to nail down factual information.
Fill tank to click-off
Reset trip meter
Reset AVG MPG
Drive most of the tank out, down to 1/4 tank remaining or even a bit more

Refill tank to click off and record gallons pumped
Record trip meter distance driven.
Take your phone and punch into the calculator the total miles, hit divide, punch in gallons used to refill.
This is your actual MPG
Reset trip meter.
Reset average MPG.

Rinse and repeat every tank full and see how interstates blow goats and suck your fuel with bigger tires and gears in ways you didn't expect while you see that cruising level roads around town at 40 MPG gets you near stock MPG (around 17-18 if your really light footed).

 

I understand that. The reference to the mileage posted here was just from yesterday. I've been tracking the mileage for a year, and never been off by much more than 1MPG from what the computer is saying. My concern, again, was this: If the computer is telling me I'm getting more MPG than I'm actually getting, AND the speedo is spot on (which it is), unless it's some programming error from Jeep, it would be racking up miles too quickly on the odometer (it isn't).

What is perverse to me is the idea of having to "tinker" with the measurements. This is mathematics: the formula to determine RPM to mileage is a mathematical reality. My new question is, which is screwed up, Jeep's programming or Superchips?

I neglected to mention that I did the measured mile yesterday (cruise set at 60, 1 mile on I-95, with a stopwatch) -- DEAD on. I typically refill at a half-tank, so this is purely out of interest, not necessity. What I seem to have found is:

• Odometer is reading accurately;
• Speedo is reading accurately;
• something is likely afoul with the calculations of fuel economy, though my manual calculations are typically over 20MPG;

As I continue to mess with this, I have to wonder if somebody has "tinkered" with the engine to make it into a unicorn, after all.
One thing I have NOT mentioned in this thread: This vehicle originated as a Canadian vehicle.

I plan to redo the measured mile again today (just to be sure) and go from there.

My lone concern at this point: I do NOT want the odometer reading thrown off by the tinkering.

 

Yes, thanks. It ALL helps. Information is king.

 

More Data:

Just returned from another GPS/stopwatch measured mile.

GPS speed: 60 MPH (61 at one point on a slight down hill)
Cruise Control: 60 MPH
Measured mile: 60 seconds, exactly.
Measured mile odometer: 1.0, exactly.
Set tire size: 32.5
Actual tire size: 315
Measured actual tire size: 33-5/16

Conclusion: Speedo is SPOT on. Odo is SPOT on.
Calculated fuel by computer: 22.1 MPG

Next step: Burn through a tank of gas and measure it out. Looks like I've got some driving to do. See you in a few hours.

 

Sounds like it must be a magical combination of driving style and other driving conditions like route that results in the mythical capability to get >20mpg.

It's unlikely it's anything that special about the particular Jeep.

But it sounds like you've gone to sufficient effort to verify the odo and speedometer are correct.

BTW I think the odometer and speedometer on at least my 2015 JK are not synchronized. I think the speedo is intentionally programmed to read a little bit faster than actual speed, but the odometer doesn't seem to have the same bias. I don't have hard data to back this up (I did, back when I measured). I have opted to get the speedometer right and let the odometer read a hair slow.

 

did you verify your programmer changed the gear ratio? I would double check that and verify you have 4:10's by jacking a tire off the ground and put t-case in neutral and count the turn on your driveshaft.
What air pressure are you running?

 

I have a hard time imagining 22mpg on a 315mm wide tire.

My 2 door is pretty heavy with bumpers/tires/sliders/driver with 35x12.5r17 tires and I tend to average closer to 15MPG on the highway and closer to 17MPG putting around the city. If you live in a flat area, close to sea level and very low levels of ethanol in your gas then I suppose I could see a 22MPG economy, but it just won't happen here in the mountains.

 

Alrighty, kids.

Mileage: 209.8
Fuel: 9.33 gallons

Verdict: 22.49 MPG
Computer: 22.0 MPG

 

when you do the mile check does your odometer match.