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  #1  
Old September 12th, 2020, 18:58
Grey Crawler Grey Crawler is offline
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Poor gas mileage help

I have a 1998 xj with a 4.5"lift. AW4 auto with 3.55 axle gears and running 32" Cooper STT tires. I am lucky if I get 180 miles out of a tank. My last xj was also a '98, stock with AX-15 and 3.07 gears and 29" tires. It would easily get 280 miles every tank of fuel. I know I would lose some mileage with the auto and bigger tires, but 100 miles per tank seems excessive. I have changed o2 sensors, map sensor and switched to 4 hole injectors with no difference. I also have the cat delete. Any advice?
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  #2  
Old September 12th, 2020, 19:23
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Re: Poor gas mileage help

check your speedo with GPS...there are apps to do this with

My stock 96 speedo was off by almost 10%, which is why I thought I was getting great mileage.

I checked it to see how far off the speedo was with 31" tires...and found out it was off by 1 mph @ 60.

BTW, the lift hurts MPG as much as the bigger tires...
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  #3  
Old September 12th, 2020, 20:16
Evan03 Evan03 is offline
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Poor gas mileage help

Grey crawler.
I get about the same out if a tank.

I pack fuel if going very far. Im on 33s and 3.55 with the ax15

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  #4  
Old September 13th, 2020, 06:14
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Tim_MN Tim_MN is offline
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Re: Poor gas mileage help

How many slices are in a pizza ? How many miles per gallon is the correct unit of measure, not miles per tankful.

As suggested already you need to adjust for any speedometer/odometer error from the larger tires, and to expect lower mpg's because the axle gear ratio is so far off for 32's. Not having a cat converter installed may also be part of the problem, you should fix that.
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  #5  
Old September 13th, 2020, 12:13
Agreen Agreen is offline
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Re: Poor gas mileage help

The brand of O2 is important. NTK is the OEM supplier and it has been proven time and again that other aftermarket brands don't deliver quite the same results.

An exhaust leak will also drain your mileage. If the manifold is cracked, your mileage will suffer worse than you think it will.

An ignition upgrade can help a little and improve longevity of the wearable components. A cap/rotor with brass (vs aluminum) contacts helps mileage a little.

The 4.5" lift and 32" tires don't help either, but as was said before, compensate your odometer. If your stock tire size was 28.3" and now you're at 32", there's a difference in circumference. The 28.3" tire rolls 88.8" per revolution, and the 32" tire rolls about 100.5". Divide 88.8/100.5 and you get 113. Multiply your miles by 1.13 and you're at 203.4 miles. Let's say you use 16 gallons (about my average) and you're pretty close to 13 mpg, which ain't terrible for a lifted jeep.

But to get more accurate, do like the above said and use a GPS speedometer to verify your original speedometer is accurate too. (The speedometer drives the odometer, btw)
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  #6  
Old September 14th, 2020, 12:27
Grey Crawler Grey Crawler is offline
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Re: Poor gas mileage help

I had actually checked both jeeps with my gps to see how accurate they were, and both were pretty good. Installing a cat and doing a tune up are both on the to do list. My buddy had a similar set up to mine on his 2001 xj and he was surprised at how bad my mileage is. Hopefully I can find something that helps. I know that going by miles per tank isn't the most accurate, but same jeep, same engine, same size tank. Losing 35% is a big hit. I'm not ready to accept this yet. Lol. Thanks for the suggestions
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  #7  
Old September 14th, 2020, 19:57
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Tim_MN Tim_MN is offline
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Re: Poor gas mileage help

Once you have accurate MPG records for some significant mileage, you can determine if your MPG's are low.

You are using NTK O2 sensors, NOT Bosch or parts store generic O2's, and have inspected the O2 wire harness for damaged wires or chafed insulation, the O2 wire plugs for bent/pushed back wire pins, and the O2 sensor fuses ?


The primary suspect for a 30%-40% drop in mpg's is O2 sensor issues.
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  #8  
Old September 14th, 2020, 20:40
RCP Phx RCP Phx is offline
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Re: Poor gas mileage help

I don't know any way to get the correct mileage without doing a speedometer correction. That being said it all depends on what circumstances your doing your testing under. When my Jeep was almost new I got just over 350 miles on a full tank. Now with the stroker I can get almost 18 mpg if I'm gentle with the skinny pedal go as low as 13.8 mpg if I'm heavy footed or climbing a long hill.
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  #9  
Old September 15th, 2020, 13:25
Evan03 Evan03 is offline
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Re: Poor gas mileage help

I found no noticable difference with mpg with 02 sensor unplugged or plugged in. No check engine light either maybe something is wrong with mine.
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  #10  
Old September 15th, 2020, 14:01
alwayzAMC alwayzAMC is offline
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Thumbs up Re: Poor gas mileage help

did you unplug the front O2, or the rear one behind the cat? the rear one will not give any information to the computer that will change the way it runs, it is for cat efficiency and i believe it only trips a light if there is no noticeable O2 change between the front and rear O2, it is a way to check for cat efficiency. my O2 is zip tied above my exhaust and reads ambient O2 levels, and trips no codes.

if i am on highway i get 12 and 15 mpg in the city, if i go by speedo info, my speedo is close to accurate at 20 MPH, at 55 i am going 62 at 70 i am going 80, at 75 i am going 86 at 90 i am over 100, 104 i think? cant go that fast anymore due to worn steering parts. i guess that is a long way to say there is no way to test your mpg using your speedo or miles per tank based on your dash info.

if you really want to test your mpg fill your tank, follow a gps route that has the mileage figured out for you and totally ignore what ever your dash says, refill your tank and use the mileage on your gps. i have a 99 auto with 34;s and 355 gears also. i average 16-17 mpg. or so i think. i have not set up a new speedo gear as i am still going to regear to 456
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  #11  
Old September 15th, 2020, 14:07
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Re: Poor gas mileage help

or use a speedo app on your phone, drive a few miles at a constant speed (I used 60 mph), and see what you speedo says.....then compare....can even do it for several miles in a row as long as you maintain constant speed

IF you use 60 mph, then you can divide 60 mph GPS by what your speedo says, and subtract 1...that leaves the % it is off.



Or use the mile markers on the highway.....do a constant 60 mph by your speedo, and time how long it takes to go 1 mile. Then, divide 60 by the time in seconds, and subtract 1, what's left is how far off it is in %.
(60 mph by speedo takes 50 seconds...so 60/50 is 1.2, which means it is off by 20%, and 60 speedo mph is really 72 mph)


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Last edited by TRCM; September 15th, 2020 at 14:14.
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  #12  
Old September 15th, 2020, 17:36
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techno1154 techno1154 is offline
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Re: Poor gas mileage help

WOW! You guys speedo are really far off. My speedo is slow by 7% and I think it is unacceptable.

I use a stand alone Garmin GPS. I clear it for miles travelled before hitting the highway. I set the cruise control for my preferred speed and compeer the GPS speed with what is shown on the speedo. At the end of the trip/day, I also compeer the the GPS distance to the speedo distance. That procedure is about 99% accurate.
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1996 XJ; 4.0L; AW4; NP231; SYE; D30 and D44; ECTED; 4.10 gears; 30X9.5 -15 BFG/AT/KO; 3" lift; Rusty's LCA; JKS ADJ UCA; Kevins ADJ Track bar; Drawtite Front Receiver; Dual Electric Fans; Dual Battaries.
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  #13  
Old September 15th, 2020, 18:50
alwayzAMC alwayzAMC is offline
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Re: Poor gas mileage help

is it normal for the speedo to be off progressively worse as your speeds go up ? i would have thought it would be a constant percentage, but am finding that is not the case with mine.
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  #14  
Old September 15th, 2020, 19:55
Evan03 Evan03 is offline
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Poor gas mileage help

Mines a renix I only see 1 on sensor on it

Im off 5-7 with 33s and 3.55 gears.

With 307 im off 12-15 speedo reads slow.

I also use GPS to kow actual speed.
On my pickup I calibrate computer for tire size tht would be cool option on old jeeps im sure the new ones can.

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  #15  
Old September 16th, 2020, 09:48
Black1990jeep Black1990jeep is offline
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Re: Poor gas mileage help

Keep in mind that the speedometer may be accurate, or not, but that does not mean the odometer is likewise.

the speedometer is driven by a spinning magnet pulling on the needle movement, excess friction in the needle bearing can cause an inaccurate speedo reading.

the odometer however is direct gear driven by the cable, and thus does not necessarily behave as the speedometer as for accuracy the speedo can be way off do to needle friction, yet this will have no effect on the odometer.

to test your odometer, you need a known marked length run, preferably several miles to assure precision. likewise a gps can be used to check the odometer.
the speedometer may be wrong and odometer correct, or the speedometer maybe correct and odometer wrong, or they can both be wrong, or both correct.

the fact that the speedometer calibration has been found by some to be not proportionality off shows how the speedo can be wrong in a way that an odometer cant be as the odometer is gear driven, so it MUST read proportional to the drive cable turns, where as that is NOT the case for the speedometer.

So calibrate the odometer for gas mileage determination, do NOT assume that a correct reading speedometer means the odometer is correct, and visa versa.
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