Average 8 mpg, need suggested fixes

TJ4Jim said:
The question was asked earlier but again did you correct your speedo for the tire size.
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No, I did not. I know that I am travelling farther per rotation on these larger tires than stock, BUT, when I figure the mileage, I should be getting more that 8-9 (which actually may be 9-10 since the tires are larger in circumference). If I still had the original size tires on the vehicle, I would still have a problem.
 
Clarythedrill said:
No, I did not. I know that I am travelling farther per rotation on these larger tires than stock, BUT, when I figure the mileage, I should be getting more that 8-9 (which actually may be 9-10 since the tires are larger in circumference). If I still had the original size tires on the vehicle, I would still have a problem.
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Correct your odometer before chasing demons any further.
 
CharlesHS said:
THANKYOU....
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Charles, read the post directly above yours. No, I have not.

My jeep has no idea what tires are on it. It assumes it is stock, and therefor travels X distance per rotation, and then posts miles driven. Although I put larger tires on, the jeep doesnt know that. I will admit that I am most likely getting a little more than 8-9, but when I hand figure the mileage, I should show more on paper than that if the vehicle is running optimally. That is my problem, figuring out what is not optimal mechanically.
 
Clarythedrill said:
Charles, read the post directly above yours. No, I have not.

My jeep has no idea what tires are on it. It assumes it is stock, and therefor travels X distance per rotation, and then posts miles driven. Although I put larger tires on, the jeep doesnt know that. I will admit that I am most likely getting a little more than 8-9, but when I hand figure the mileage, I should show more on paper than that if the vehicle is running optimally. That is my problem, figuring out what is not optimal mechanically.
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IF your odometer says you drove 220 miles then more realistically you drove about 245 miles and you add 11 gallons of fuel; there is a difference 20 mpg vs 22 mpg...
 
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CharlesHS said:
IF you odometer says you drove 220 miles then more realistically you drove about 245 miles and you add 11 gallons of fuel; there is a difference 20 mpg vs 22 mpg...
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Charles, I understand what you are saying, really I do, and appreciate your comments. IF I was to put stock size tires on, I would still be getting 8-9 MPGs when going off of the odometer VS gallons added. That is the issue. The larger tires are IN REALITY (I do realize this) giving me more. BUT, when hand figured and I come out with 8-9, there is a mechanical issue, not speedo issue. If the jeep should be getting 15-16 if running optimally with stock tires, why am I reading 8-9, as that is what the jeep thinks is on the rims?
 
OK.. Enough on that specific way to increase MPG.
The next thing to address is the old OEM fuel injectors.
K Suspension sells a great set of matched 12 hole fuel injectors that will add miles to your tank full for about $150.

K Suspension

Others have posted about obstructions in the catalytic converters that will affect gas mileage and engine power.

I know you are getting a lot of suggestions on what to do... address the ones you can as you can...
 
Calculator cares by about 10%. Wind resistance also cares, if speedometer indicates 70 you could actually be traveling closer 80. The faster you drive the worse mileage you'll get too.

That may not be all your issues but it's definitely not something that should be ignored completely.

Also, I wouldn't expect to get the claimed 18-20mpg that some people say they get. I regularly get between 11.5-13.5 mpg.
 
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xxdabroxx said:
Calculator cares by about 10%. Wind resistance also cares, if speedometer indicates 70 you could actually be traveling closer 80. The faster you drive the worse mileage you'll get too.

That may not be all your issues but it's definitely not something that should be ignored completely.

Also, I wouldn't expect to get the claimed 18-20mpg that some people say they get. I regularly get between 11.5-13.5 mpg.
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While he wont get 18-20 mpg due to his modifications and weight; I consistently get 18-20 mpg on the highway in my almost stock TJ and when I drive outside California the gas mileage jumps 2 mpg....

The point I am trying to make is that there are simple things that can be done to increase the MPG after he figures out why his MPG is presently so low.
 
My bone stock TJ Sahara 150K Mi. gets 17-18 on the hwy.
I believe 8 mpg should make for black exhaust pipe and raise hell with the cat, stick your finger in the exhaust and rub the ID is your finger black?
I also agree on the odometer being off. Confirm the accuracy of it using a GPS.
 
If he is unwilling to use actual numbers instead of the (known) wrong numbers from larger tires there is little point trying to help him. Until he recognizes that larger tires throw off mph and odometer any 'hand' math is worthless. Larger tires affect mileage. Higher lift affects aerodynamics which affect mileage. If he is using ethanol fuel that affects milage. Running larger tires throws off gearing. This makes engine run in less than ideal rpm range. This affects milage.
 
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Goatman said:
If he is unwilling to use actual numbers instead of the (known) wrong numbers from larger tires there is little point trying to help him. Until he recognizes that larger tires throw off mph and odometer any 'hand' math is worthless. Larger tires affect mileage. Higher lift affects aerodynamics which affect mileage. If he is using ethanol fuel that affects milage. Running larger tires throws off gearing. This makes engine run in less than ideal rpm range. This affects milage.
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His math is completely solid.
You hand waving isn't helping.
I hope people stay on track with useful suggestions. Discussing gearing or the speedometer isn't helpful at this point.
 
pagrey said:
His math is completely solid.
You hand waving isn't helping.
I hope people stay on track with useful suggestions. Discussing gearing or the speedometer isn't helpful at this point.
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Math based on completely wrong numbers is anything but solid.

Fixing the speedo and getting accurate numbers is the most useful suggestion in here. 8 - 9 could easily become 11 - 12 which depending on his driving style and average speed could be perfectly fine.
 
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My stock Rubicon was getting about 10mpg with old cats and the Bosch plugs that came with it. I don’t know what each accounted for, but after changing both I went up to 14mpg. The cat wasn’t throwing it’s own codes but I was getting O2 sensor codes.

There are ‘MPG Threads’ here where most people say they’re getting around 12 mpg. This is the first thread I’ve seen where the majority seems to be anywhere close to 18.

Is it possible the previous owner of OP’s Jeep changed the odometer or gearing for larger tires?
 
The OP’s logic is that he’s calculating and talking about ”theoretical” MPG using OEM tire size, and he believes his ‘actual’ MPG should be better since the wheels are bigger, and therefore covering more distance per revolution. He’s trying to understand why his ‘theoretical’ MPG is so low.

But bigger wheels are actually less efficient. They cover more ground per revolution, but take more energy to accelerate/decelerate and wheels are almost never traveling at true constant speed, and therefore use a lot more energy. There’s a reason high efficiency cars have small wheels! So without more data nobody can really say if the MPG should be more or less than the theoretical value.

So it’s not that the ’math’ is bad, it’s that the logic is built on an invalid assumption, which makes the numbers used pretty much meaningless - there’s no real starting point. So people are saying this effort is pointless until there is data that can be trusted.
 
What's your gearing? This is subtle, but I know the first time I got my TJ I tried using 5th gear on the highway and could watch my fuel gauge drop. All the vehicles that I had ever extensively driven you stuck them in the high gear as soon as you could and never touched the stick until you were slowing down or trying to get more acceleration.

A major part of your MPG, like stated before is aerodynamics, so ensure you're actually doing the correct speed or else, MPG will plummet. That can easily be done in your head by comparing the 2 diameter of tires. I do this all of the time as well. The MPG calculation is simple to convert between tire sizes.

All the other suggestions are definitely sound suggestions.

I found the 4.0L to be a bit under-powered for the stuff I liked to drag around, so I typically saw low teens when I had it, but part of that was gearing as well. I was able to get 18+ mpg from the 4.0 and 33 x 12.5s, but I really had to try. I'm not sure why these were given 3.07s EVER though. I have a manual so I basically just ignored 5th. However I need to run mud tires to get anywhere without using the winch every 5 minutes and the TJ always got horrible MPG with good mud tires. And add in the 4lo stuff and/or towing and logistically it was smarter to tow it than drive. I could tow it for around 14 MPG or more and bring a bunch more stuff or mess with gas cans hanging everywhere and knowing my gas stations.

But I'm biased and got a TJ to swap out the engine anyway, but actually grew to appreciate the reliability and simplicity of the 4.0, but if it had had a bit more displacement (imagine ~4.9L with about 300 hp) and different gearing, without having to use premium, I would have had a much harder time swapping the engine.
 
So to top this off:

Thermostat - now changed - to the correct temp I hope

Tire size vs speed and mpg - if you're not corrected yet, how about using GPS. I'd like to think that your fuel gauge works.

No brake drag?

Possible cat coverter plug - test is usually cheap.

I also see that you changed a bunch of sensors all at about the same time - was a complete ECM reset done? This may or may not help.

Just throwing my pennies out there ;)
 
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