Would a tune or gear change help bad MPG with 35 inch tires?

[jasunto]

Your getting 28mpg on highway with 35s? Are you running a lift? I have mopar 2 inch lift and 35s on my rubicon gladiator and only get max 17 mpg on highway

14.5 average free way or highway here with 4:10 rubicon 35s and icon 2.5” lift

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[Papa-KDog425]

If the dealer added the 35’s they are required to calibrate it. That little thing called the odometer mileage statement when you buy it lol. Call them on it. I did- and they had to calibrate at no charge when I bought my Tacoma that they lifted and put 35’s on.

No, I bought it this way off the lot. It has not been programmed. It was brand new - dealer added the 35's and lift.

 

[cgflyer]

As far as calibration goes, when I ran my 35's I ended up set at about 33.8" or 33.9" for accurate speedo even though my tires measured 34.5" and with my 37's (36.5") I am set to like 35.5. If you guy a Rubicon with 33" it will likely be very close already if you add 35's...but not sure if that means they are way off when running stock 33's or if the programming is actually just hit or miss?

 

[piroman683]

Here's the high level science guys, there are 2 things to consider that impact fuel being burned. The first is engine RPM, and at highway speeds engine RPM is low (i'm calling 2650 and below low) which means torque and HP at that RPM are pretty low. (now the caveat to this is the torque curve on the 3.6 is pretty flat which is nice).
The second item is engine loading, which is how hard is that engine working at that particular RPM. For simplicity we can correlate this to throttle position since throttle position is relative to air flow and thus fuel flow. Have you even driven uphill with the pedal to the floor and not see RPM's change? That's because the engine was at it's maximum loading making the exact power it needed to propel itself forward, if RPM's decrease you down shift, if the increase you could upshift.
now that we have these two points sorted out (again high level view of things) lets now consider stock configuration vs. 35in tires (no regearing).
At highway speeds with 35in tires your RPM's drop is around 8% which in turn means lower engine RPM, however the tradeoff (there always is) is that then engine must work a lot harder. Let's do a full scale example. The stock tire is 31.47in in diameter and at 75mph with 3.73 gears and the manual trans in 6th gear (.72 ratio for over drive) you have a cruising RPM of about 2150 RPM's. This, mind you is at a pretty low engine loading, meaning you are not pressing down on the pedal very far to maintain speed.
Now you've upgraded to those beautiful 35's (in my example here I'm using K02) which, for the 315/70/17 version, are 34.4in in diameter. Leaving everything else the same your RPM's now degrease to 1970, close to an 8% drop in RPM. However, you now need the engine to work harder, meaning more pedal to the floor, significantly more than with the slightly higher RPM's of the stock configuration. This means burning more fuel! How much fuel you ask, well that I'm not 100% sure of as I dont know the exact engine loading. But lets look at that in another simplified way. If in stock config you are only loading the engine 15% we can just simplify fuel burn in the following way: engine RPM*loading=fuel burn. So, 2150*.15=322.5 "fuels burned". Now, at 1970 lest first assume same engine loading so we get 1970*.15=295.5 "fuels burned". Ok it looks like you are saving, but in reality the engine does need to work harder more of the time. In fact, we are burning more fuel as soon as the engine loading exceeds 16.3%. I achieved this by taking the stock "fuels burned" and dividing it by our new RPM (1970) to see what the equivalent engine loading would be, which is 16.3%
Now a regear will reduce engine loading, but also slightly increase RPM. For my truck I regeared to 4.56 so I'll use that here. at 75mph RPM's are roughly 2405 vs. stock at 2150. I'm in a slightly higher power band which makes it easier for me to maintain consistent engine loading. Once again let's first assume 15% engine loading. 2405*.15=360.8 "fuels burned", pretty big increase. Now in order for me to have the same "fuels burned" as stock I need to determine what engine loading I need to achieve 322.5, this comes to be 13.4%. What does this mean? well, we know with the better gear ratio the engine doesnt have to work as hard as long as the stock engine does. We also know that even if we have stock gears but bigger tires our "fuels burned" savings go away as soon as we load the engine more than 1.3% over stock. Because of these facts (engine loading and RPM) there is always a little trade off between how you run gears and tires. In the end you could expect, at best, with the right gear and tire selection, theoretically match OEM MPG. However, i have not factored in here the extra mass associated with tires, roll resistance etc. Those all require more engine loading with stock gears. (now we get into physics such as F=1/2mv^2, coefficient of friction, drag, etc. - but we are not getting into that shit here).

Summary of the summary - gears will help alleviate some of the poor MPG, there are a ton of factors that impact how much is impacts the terribleness of your MPG, but it helps. Tunes don't do shit anymore. These engines are well tuned from the factory, you can even look at current exhaust mods out there and they don't really do shit for performance, and definitely not MPG.

 

[salvino]

Your getting 28mpg on highway with 35s? Are you running a lift? I have mopar 2 inch lift and 35s on my rubicon gladiator and only get max 17 mpg on highway

I'm running the Mopar lift and 37s with 4:88s and got 17.4 average from Seattle to Tucson. The right gears help.

 

[KurtP]

One of my pet peeves is things having corners cut. I cant understand why someone would lift and big tire a jeep and not consider regearing.

(new tire size x factory gears)/factory tire size = new gear normal

 

[Helryd68]

There is no way you could get 28mpgs on 35's with any gearing. Maybe 20-21 at the most. I don't like to call BS but that's what it is.

As far as the question being asked, tuning isn't going to anything for Gas Mileage, these things are tuned and setup from factory to get you the most out of your jeep you can get. If you put larger tires on mpgs are going to take a hit and with 3.73 gearing on 35's your going to see a nice hit. Regearing to 4.10 or 4.56 would get you closer to stock mpgs. That's the main reason I opted for Max Tow, I get pretty much 18.2-19.1 mpgs depending on how far I drive mine and if I stay under 80 mph. That's with it sitting like this

I’ve got 35’s and a 2 1/2” lift and I am getting 13-15 mpg with a 4.10 gearing. I would love 18-19. What am I doing wrong? I have the taser mini, should I also have the dealership adjust for my tires? Obviously with the taser removed.

 

[KurtP]

I’ve got 35’s and a 2 1/2” lift and I am getting 13-15 mpg with a 4.10 gearing. I would love 18-19. What am I doing wrong? I have the taser mini, should I also have the dealership adjust for my tires? Obviously with the taser removed.

Regear and a pulsar.

 

[Jeeperjamie]

I’ve got 35’s and a 2 1/2” lift and I am getting 13-15 mpg with a 4.10 gearing. I would love 18-19. What am I doing wrong? I have the taser mini, should I also have the dealership adjust for my tires? Obviously with the taser removed.

Not sure, I'm using the JScan I didn't have the dealership flash mine. Is yours a Max Tow or Rubicon because it seems like that matters. Also my 35's only weigh 63lbs a piece and I'm running my stock steely wheels. 4.10 on a 8spd should be plenty for 35's.

 

[cgflyer]

I’ve got 35’s and a 2 1/2” lift and I am getting 13-15 mpg with a 4.10 gearing. I would love 18-19. What am I doing wrong? I have the taser mini, should I also have the dealership adjust for my tires? Obviously with the taser removed.

I think again, this conversation goes to driving style, road and weather conditions and maybe how you are actually calculating your mpgs. Before the Pulsar, I was getting 18 in my Rubi with 35's and no lift...I added the Pulsar pretty quick and bumped to 19+ fairly consistently until I added the full height bed rack. I also have Gibson catback, throttle body spacer, Metal Cloak fenders, a custom rear bumper with dual swing outs, front bumper and winch and rooftop tent...I'm heavy. With 37's and 3.5" lift, stock gears and Pulsar, I'm 17.5-18.5 mpg on the highway. I really don't think city driving is a good indication of what your vehicle is capable of for mpg as there are just way too many variables....also, do you have remote start and do a lot of idling?...the only true test of mpg in my opinion is what has been discussed in many threads...fill up at the same gas station every time, don't top off, take the same exact route every day and fudge for weather (wind) and manually calculate your mpg...

 

[93civej1]

315/70/17 KO2 here and corrected using JScan. I am getting 18ish with normal driving.

 

[tysongladiator]

I've got an overland, 3.73s, on 35s. I also have a friend with a max tow on 35s. We get about the same gas mileage. Around 17mpg. I had a buddy with an issue and he had to drive i limp mode and I had to follow. We were going about 60mph and I got 17mpg. On the same 35s with the 3.73s.

 

[Jeeperjamie]

I've got an overland, 3.73s, on 35s. I also have a friend with a max tow on 35s. We get about the same gas mileage. Around 17mpg. I had a buddy with an issue and he had to drive i limp mode and I had to follow. We were going about 60mph and I got 19mpg. On the same 35s with the 3.73s.

Limp mode want allow you to go past 40mph, how were you driving 60 in limp mode just curious. 19mpgs on 35's and 3.73 gears, you would be the first to report that.

 

[tysongladiator]

Limp mode want allow you to go past 40mph, how were you driving 60 in limp mode just curious. 19mpgs on 35's and 3.73 gears, you would be the first to report that.

My friend was in limp mode in his JKU and I was following him in my JT. The system was having issues the entire trip. At one point, he could only go in 1st gear, then it will allow him to speed up, and then go into limp mode will he was driving and he had to slow down to about 30mph. He was on the radio letting me know what was going on.

We did have a good run for about 30 minutes where we went about 65mph, but then it kicked back in and we actually had to pull over to the side of the road. Lights flickering on the dash, levels changing, it was ugly for the 3hr drive.

 

[cgflyer]

Limp mode want allow you to go past 40mph, how were you driving 60 in limp mode just curious. 19mpgs on 35's and 3.73 gears, you would be the first to report that.

Limp mode limits you to 3,000 rpm, not speed...been in limp mode several time in this vehicle

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