Is dropping 5-6 miles per gallon normal?

[Bailey525]

So I posted recently, I have a 4 door 2018 JL 4 Cyl turbo and I had to replace the stock 245/75/17 Michelin Highway tires. I just went to a slightly bigger A/T tire in the 255/75/17 size. I recently took a 4 hour road trip and only averaged 19.6 mpg. I took the same trip about 6 weeks ago and got roughly 26-27 mpg. I do have a heavy foot but that’s been the same with both sets of tires.

I’m using the same 89 octane gas as well. It just seems like a drastic drop off in mpg for just a slightly bigger tire..

Any similar experiences??

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[mwilk012]

Was the wind blowing harder? Were you driving faster?

 

[bajanut]

Hey I put a rack on my jeep and the mileage dropped 3 to 4 miles !! and just the rack no basket

 

[Bailey525]

Was the wind blowing harder? Were you driving faster?

Was the wind blowing harder? Were you driving faster?

I went on a backpacking trip and it was 60+ mph winds in the mountains after the front passed but from what I recall, the wind wasn’t an issue on the drive up..I drove same speed, between 70-85 mph..

 

[Heimkehr]

Kevin,

Anecdotally, the Wrangler's fuel efficiency has been known to noticeably drop off when the vehicle is driven at sustained speeds over 70 mph or so. If this was your habit before the ATs were installed, though, and in similar conditions (elevation, etc.), possibly there's more that needs reconciling here.

Have the air filter and spark plugs been changed according to the schedule evident in the Owner's Manual? And et cetera.

Possibly the 60 mph winds are the culprit. I'd personally monitor the average mpg for at least two full tanks of fuel before determining that the 5-6 mpg reduction wasn't an aberration. On that point, is there any record of CT using winter-blend fuel that would adversely affect such calculations?

 

[Bailey525]

Kevin,

Anecdotally, the Wrangler's fuel efficiency has been known to noticeably drop off when the vehicle is driven at sustained speeds over 70 mph or so. If this was your habit before the ATs were installed, though, and in similar conditions (elevation, etc.), possibly there's more that needs reconciling here.

Have the air filter and spark plugs been changed according to the schedule evident in the Owner's Manual? And et cetera.

Possibly the 60 mph winds are the culprit. I'd personally monitor the average mpg for at least two full tanks of fuel before determining that the 5-6 mpg reduction wasn't a one-time event.

I will definitely do that. Yes, my highway driving is the same, I am due for an oil change and scheduled maintenance soon so I will see where it stands after that. Should there be a difference between 87-89 octane?

 

[Heimkehr]

Should there be a difference between 87-89 octane?

None, in my experience.

...nor should an oil change being due affect fuel efficiency.

 

[Carlton]

Did you reprogram for the larger tire size? If not, your mpg is incorrect.

 

[Dkretden]

Did you reprogram for the larger tire size? If not, your mpg is incorrect.

This.

 

[rohdawg]

I know Connecticut switches to winter blend on the 15th of September. Was your first trip prior to the change?
Winter blend and needing to reprogram for your tire swap may be the culprits.

 

[Bailey525]

Did you reprogram for the larger tire size? If not, your mpg is incorrect.

Honestly, Firestone put the tires on so I assume they did everything correctly. How would I know or check that? Is that necessary going from 245/75/17 just up a notch to 256/75/17??

 

[Dyolfknip74]

Honestly, Firestone put the tires on so I assume they did everything correctly. How would I know or check that? Is that necessary going from 245/75/17 just up a notch to 256/75/17??

The tire shop would not have reprogrammed for bigger tires. You need some hardware to do that.
You have some options. I personally have a SGW cable and use the JScan app. Some people use the Tazer. Some use a Procal. All up to you but it needs to be done because they 100% didn't do it.

 

[Carlton]

Honestly, Firestone put the tires on so I assume they did everything correctly. How would I know or check that? Is that necessary going from 245/75/17 just up a notch to 256/75/17??

Tire shops can't reprogram. You have to buy a device to do it yourself, or pay the dealership to do it. Until it is done, your speedometer, odometer, and mpg will calculate incorrectly.

 

[Bailey525]

Tire shops can't reprogram. You have to buy a device to do it yourself, or pay the dealership to do it. Until it is done, your speedometer, odometer, and mpg will calculate incorrectly.

Hmmm, I had thought because I was onky going up one size that I wouldn’t have to do it, but what you say makes sense. I’m clueless with this stuff so if I go to dealer do you know ballpark what it should cost? I have a 2018 Unlimited 4 cylinder turbo. Thanks

 

[Dkretden]

Hmmm, I had thought because I was onky going up one size that I wouldn’t have to do it, but what you say makes sense. I’m clueless with this stuff so if I go to dealer do you know ballpark what it should cost? I have a 2018 Unlimited 4 cylinder turbo. Thanks

tiresize.com puts it at a 1.9% difference in diameter/circumference.with 12 revs per mile less than your stock tires. You will have about a 1-2mph speedometer error.
https://tiresize.com/calculator/

that change alone doesn’t seem to me to to be enough to account for 5-6 mpg difference but, until you reprogram, you won’t know.

Dealers can charge what they want for reprogramming but I believe that the cost is ‘normally” around $125. Some on these forums have said more some less…… you can just call your dealer and ask what the cost is for reprogramming for larger tires…..

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