What's your MPG with large tires and re-gearing?

jeep_boss

jeep_boss

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In the ~6 months that I have had my 5.13 geared 05 TJR (6spd manual) on 35" tires, my MPG has averaged:
  • 12.3 (city)
  • 14.6 (highway)
That said, I just replaced my exhaust manifold that had clear signs of an exhaust leak, due to missing and untorqued bolts. I'm wondering now if my MPG will increase with a new (much closer to stock) exhaust manifold, with all of the bolts torqued to spec now.

FWIW, my Jeep seemed to be "happier" from both a performance and MPG perspective with 89 octane, which I have ran on occasion.
 
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With 33's, at 26psi, 5 speed, 4.56 gears and the correct speedo gear, I average 13.7 mpg on 87 octane fuel and the 4.0L purring like a kitten.
 
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I get 13 in the city. 2003 sport, 4.0, 5 speed manual, 4.56 gears, rear locked and 33” tires. Usual stuff on it, aftermarket bumpers, winch etc. Engine runs great. Just trash for mileage. But, I knew it was going to suck and bought it anyways. I’m sure if I shed some of the weight like bumpers and winch it would be better. But, it makes me happy.


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13.1mpg city on i6 4.0, 5 speed manual, 33x10.5x15 toyos, overgeared at 4.88 (Have correct gear driven speedo gear in as well). Currently have a significant exhaust leak and a severely cracked header. Hoping a replacement of one in the future will help fuel economy.
 
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4.0 5sp 5.13s on 35s avg 13.6mpg. 90% backroad, 10% small town. Backroad pretty steady 55mph, in town 35mph with 2 lights. Rarely hit 60mph.
 
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2.5, 3.73:1 gears, automatic, 31's (but when i measured them they were actually 29.5's). Roughly 50/50 highway and town, never had it over 70.
The best so far (had it 4 months) is 14
However, just installed the CORRECT oxygen sensor (NTK part #23506) ... can already tell better running engine, over entire rpm range, hope to get better mileage now
 
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KCsTJ said:
2.5, 3.73:1 gears, automatic, 31's (but when i measured them they were actually 29.5's). Roughly 50/50 highway and town, never had it over 70.
The best so far (had it 4 months) is 14
However, just installed the CORRECT oxygen sensor (NTK part #23506) ... can already tell better running engine, over entire rpm range, hope to get better mileage now
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You sure? I thought all SEs came with 4.10s with a clutch and 4.56 with the automatic.
 
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Well it's pretty early to be positive, but I anxiously drove out to the mountains today for both a temporary break from the chaos and to field test my new exhaust manifold / O2 sensor replacement job .

Did my usual route, with an added ~20 miles over my norm for this destination, and I pulled into my driveway with the needle right on the 1/2 tank mark. I would have normally come back with a litte over a third of a tank, so I'm hopeful that my next trip to the gas station will show some sort of improvement... it's certainly "looking" that way!
 
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jeep_boss said:
FWIW, my Jeep seemed to be "happier" from both a performance and MPG perspective with 89 octane, which I have ran on occasion.
Click to expand...
Octane's sole purposes are to make gasoline harder to ignite, make it less likely to self-ignite from compression, and to slow down its burn rate. Using a higher octane fuel than recommended will not help mpg in any way and it will not/cannot help performance with an engine not designed for it.

Additional octane is added only when an engine has a higher than normal amount of compression. The additional octane serves to prevent the gasoline from igniting prematurely from the higher compression level which causes knocking and/or pinging. By itself more octane does not add power. It just stops the knocking/pinging by stopping the gasoline from self-igniting prematurely in a high compression engine.

The only positive thing running a higher than required octane does is to put more $$$ into the pockets of those in the gasoline business. Save your money. :)
 
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Jerry Bransford said:
Octane's sole purposes are to make gasoline harder to ignite, make it less likely to self-ignite from compression, and to slow down its burn rate. Using a higher octane fuel than recommended will not help mpg in any way and it will not/cannot help performance with an engine not designed for it.

Additional octane is added only when an engine has a higher than normal amount of compression. The additional octane serves to prevent the gasoline from igniting prematurely from the higher compression level which causes knocking and/or pinging. By itself more octane does not add power. It just stops the knocking/pinging by stopping the gasoline from self-igniting prematurely in a high compression engine.

The only positive thing running a higher than required octane does is to put more $$$ into the pockets of those in the gasoline business. Save your money. :)
Click to expand...

I understand combustion engines and octane, what I was saying there was with the Banks manifold in all its glory (leaks, old O2 sensors, and whatnot) my Jeep would produce more power and run "cleaner" when I ran a higher octane fuel. I would also gain an easy 1.5 MPG with higher octane.

Now I get that there are folks that swear up and down that some things just aren't possible with the 4.0L motor and everything is very static with it. Being a veteran engineer and a newcomer to this motor, all that I can do is collect data that my vehicle produces, overlay any changes to normal operations, and analyze those results to determine what improved or diminished performance.

That said, it is entirey possible that my Jeep's ECU adjusted for the higher octane and with the exhaust manifold leak, O2 sensor anomalies, and whatever else was outside of notmal operating parameters to produce what I recorded numerous times to be a positive gain in MPG. Anyone can argue against it not being possible, it is completely irrelevant to me, because I know precisely how much was gained over a half of a year's time.

The loss that I have inherited with the new, closer to OEM intake in low end torque is another story. I cannot say whether it was the Banks headers or the Banks + leak that bought it the sharper, noticiable power that it had. All that I can say is that I am satisfied with the results that I am experiencing with it now. It was a lot of work for me to get it from where it was to where it is now, my team and I were laid off from employment and I worked through two powerful winter monsoons in my driveway over the course of the last couple of days.
 
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jeep_boss said:
I understand combustion engines and octane, what I was saying there was with the Banks manifold in all its glory (leaks, old O2 sensors, and whatnot) my Jeep would produce more power and run "cleaner" when I ran a higher octane fuel. I would also gain an easy 1.5 MPG with higher octane.

Now I get that there are folks that swear up and down that some things just aren't possible with the 4.0L motor and everything is very static with it. Being a veteran engineer and a newcomer to this motor, all that I can do is collect data that my vehicle produces, overlay any changes to normal operations, and analyze those results to determine what improved or diminished performance.

That said, it is entirey possible that my Jeep's ECU adjusted for the higher octane and with the exhaust manifold leak, O2 sensor anomalies, and whatever else was outside of notmal operating parameters to produce what I recorded numerous times to be a positive gain in MPG. Anyone can argue against it not being possible, it is completely irrelevant to me, because I know precisely how much was gained over a half of a year's time.

The loss that I have inherited with the new, closer to OEM intake in low end torque is another story. I cannot say whether it was the Banks headers or the Banks + leak that bought it the sharper, noticiable power that it had.
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The 2.5 and 4.0 engines have absolutely no way to know about or adjust for a higher octane gasoline. That ability requires a knock sensor, which neither the 4.0 nor 2.5 engines have, which will adjust the timing based on the detection of knock/pinging. My BMW has a knock sensor and it will take advantage of a higher octane but the Jeep absolutely has no way, zero/nada, to take advantage of additional octane.

Neither will an 89 or 91 octane run cleaner in an engine designed for 87, modern gasolines all have similar cleaning additives regardless of the octane. That became mandated by the EPA. And in reality a higher octane is far more likely to leave deposits behind in an engine not designed for ir since it is harder to ignite and burns slower than 87 octane. 87 octane is easier to ignite and it burns more quickly than 89 or 91 octane does.
 
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13.5mpg on 33x12.5r15 with the i6, 3.73 gears, 6-speed. I averaged about the same when I was on 31x10.5r15's, though I had more power due to gearing on the 31's.
 
Go to Colorado and the octane starts at 83, then 85 and 87 is premium. The higher the altitude/elevation the lower the octane. The higher the compression thru pistons, turbo or supercharger,,, the higher the octane needed to prevent pre ignition. Not sure about TJs, but new computerized engines today can burn almost anything as the ignition is adjusted also to prevent pre ignition. But no doubt about it.... if you have no knock with 83 octane yer throwing away money buying premium.
 
I forgot to mention my mpg went from <12 mpg with 4.88 gears with 35" tires and 42RLE 4-speed automatic to 14.4 mpg on several long trips after regearing from 4.88 to 5.38. Which just goes to show engines can produce more mpg when the engine is running more efficiently as mine now does at the slightly higher rpms produced by my current 5.38 gearing. The engine runs better and is clearly happier at the slightly higher rpms as evidenced by the substantial jump in mpg.
 
4.0L, I6, 6-speed manual, 3.07 diff (I know, I'm working on that)...when I first got the Jeep, it was averaging around 17mpg with 32" pizza cutter tires. Since then, I've transitioned from 18" cast aluminum wheels to 15" aluminum alloy (about 50% weight savings), and 33x12.50x15 mud tires (MUCH, much wider than before), and my mileage came in around 14. Since I started forcing myself to do most of my driving around 2500rpm, and not allowing the engine to lug any more than possible, I've managed to push it up a tad over 15mpg.

My mileage is based on about 50% surface city roads, and 50% interstate on most given days. So, it's probably somewhere between where city and highway mileage estimates would fall.

Oh, and the 3.07 keeps me from ever seeing 6th gear.
 
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