Is dropping 5-6 miles per gallon normal?

[Bailey525]

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…..

Thank you, appreciate the tip

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[flyer92]

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…..

Apparently, dealerships are no longer authorized to reprogram for this purpose, as directed by FCA. I went to two different dealerships to confirm this, and both said the same thing. Assuming this is really the case, then we just have to do it ourselves.

 

[TheRaven]

You will definitely lose some fuel mileage to bigger tires, even slightly bigger tires. The problem in the Wrangler's case is a bunch of things - first, there is the simple fact that you are increasing final drive (and leverage ratio) which changes a bunch of things for the worse. Second, the Wrangler comes from the factory with SL-range tires, even the Rubicon. If you are upsizing you are going to at least 33" and at that size and above, I only know of two tires available in SL-range. So pretty much everyone ends up going to C-range or heavier...and that's a big difference. Third, you are raising the vehicle which changes it's aerodynamic profile. Fourth, since you are changing the final drive, you are messing with the data that the ECM uses to calculate optimal fuel mix, and the TCM uses to set shift points. These computers can't accurately determine the speed at which you are moving or the rate at which you are accelerating. So it is very important that you get the correct tire size programmed. Finally, the 3.6l and 2.0l engines are not exactly torque-monsters...so they are quite a bit more affected by driveline weight than something like my V8 Tahoe would be (or the Wrangler 392 for instance).

That said, I would expect to lose at least 2mpg in going with bigger tires. If you did like I did the first time I upsized and went with E-range tires (because that's what was available and I had no idea what it was), you could lose quite a bit more.

I feel like maybe we should sticky an explanation like this in the wheel/tire section. It seems there are a lot of these threads coming up lately.

 

[Maverick909]

None, in my experience.

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

I will contest this. LOL oil changes Do effect the MPG. ive have done this test on multiple brand trucks and cars. After 4500 miles on an oil change you will see 1mpg on avg drop and after 6500 you will see closer to 2-2.5 mpg loss on avg. The good news is it will go back up once an oil change is done with fresh oil. I change all my newer vehicles at 4500 miles now and my older truck is done at 3K. there is a 2-3 mpg drop from summer and winter blended fuel... winter is always worse though i am not sure why. ( i am not a chemical engineer lol) and depending on your vehicles timing 91oc-89-87-85 will make a difference in MPG as well. I personally found that with the 3.6L mopar jeep really enjoys 91 oct. added about 2 mpg on avg over 3 tanks of fuel over 87 oct with 3 tanks. Anyways just my notes over the last 10 years with the trucks and cars ive driven. Now as for the OP @Bailey525 head wind and tail wind at 60mph will change your MPG drasticallyso at 70-85mph your going to see big drops in mileage depending if one trip you have a tail wind and another with a head wind. Im rather impressed with your 26-27 mpg that's amazing. one other thing i did see is have you adjusted your speedo for the new bigger tire... it may not be much but 245/255 does change the computers mpg calculations

 

[Dyolfknip74]

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

You can do it yourself with a SGW cable for $30, Bluetooth or wifi OBDII dongle for $15 and the JScan app for $22. You will have a shit ton of other options it can do as well. Dealer will charge you $1xx.00 and it's a one time gig.
The hardest part of doing it yourself is connecting the SGW cable and that's not that hard TBH.

 

[Dkretden]

Apparently, dealerships are no longer authorized to reprogram for this purpose, as directed by FCA. I went to two different dealerships to confirm this, and both said the same thing. Assuming this is really the case, then we just have to do it ourselves.

Well, that’s interesting info……. First time I’ve heard that……. I believe you but I’m not sure I believe the dealers.

can others on this board confirm this?

 

[Bailey525]

I

Well, that’s interesting info……. First time I’ve heard that……. I believe you but I’m not sure I believe the dealers.

can others on this board confirm this?

contacted the dealer and they still do it, I know above poster said it’s fairly easy but I’m crazy with work and not mechanically inclined, lol, so dealer is charging $122 I believe they quoted. They will do it when I bring Jeep in for scheduled maintenance/oil change..

 

[IdahoJOAT]

Also, people underestimate rolling resistance.

Highway tires vs ATs (especially the KO2s) make a huge impact. I mean, I still think the KO2s are worth it, but to each their own.

 

[Dkretden]

I

contacted the dealer and they still do it, I know above poster said it’s fairly easy but I’m crazy with work and not mechanically inclined, lol, so dealer is charging $122 I believe they quoted. They will do it when I bring Jeep in for scheduled maintenance/oil change..

Good to hear. Maybe like many things, “California dealers’ (where the poster lives) don’t do it.

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