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Poor Gas mileage? 31s and 3.55s... I don't know where to start.

ty20404

NAXJA Forum User
Alright, so the Jeep is now my brother's but It's bugging me because I can't seem to figure out what would be causing the poor gas mileage. I'm thinking a Chrerokee w/ 31s and 3.55 gears should at least be getting 13mpg city, but my brother tells me he's only getting 10mpg.

I just replaced the TPS so I doubt that's it. I have noticed (what I believe are) some speedo gear issues at highway speeds. Could that be what's causing it? If not where should I start? It had a full tune-up about a year ago also.
 
on the tune up, did you replace the inline fuel fikter? on my 87, when we replaced it, you could hardly blow through it. try that.
 
With 31s & stock gears I was getting an average of 12-13MPG, Engine was always Lugging. With 4.11s my MPG went back up to around 17-18.
PS: I corrected the speedo so my MPG calcs are correct.
 
Slo-Sho said:
Obviously that is indicated mileage. 31's and stock gears are going to make the speedo read atleast 5mph slow if not more.

Unless he recalibrated the speedometer. If you didn't, then yes it's going to throw off your mileage numbers.

Also running 31's and 3.55's, I'm getting around 15-16mpg running around town...and most of that is sitting on 49 infront of the University in traffic. On the highway i'm seeing 17-18mpg average. Both of these are with the a/c on as well.

No check engine light or obvious signs that it might be running richer than normal?
 
UNCC_99XJ said:
Unless he recalibrated the speedometer. If you didn't, then yes it's going to throw off your mileage numbers.

Also running 31's and 3.55's, I'm getting around 15-16mpg running around town...and most of that is sitting on 49 infront of the University in traffic. On the highway i'm seeing 17-18mpg average. Both of these are with the a/c on as well.

No check engine light or obvious signs that it might be running richer than normal?

Nope, nothing... And as for the fact that it is reading under, it only reads about 3mph under, I checked it with one of those radar signs multiple times (yes I know, not a perfect indicator). But the fact that it reads a little different will not affect the mileage per tank of gasoline very greatly. I haven't done the math on paper, but in my head it would only throw off the odometer like 15 miles per tank of gas which is only going to change the MPG by about 1mpg. I'm not positive on my math, but I'm pretty sure the spedo gear wouldn't make that big of a difference. I might have to look into the fuel filter though. Because I know for sure Its only getting about 10mpg city and 14 highway. I know 4.11s would be the best way to go, but the gearing and tire size can't be bringing down the mileage this much.
 
Is your brother doing a lot of short distance, stop and go driving? If so...that'll kill the mpg's pretty quick...especially the short distance driving.

When was the last time a good fuel injector cleaner like seafoam was run through the tank and intake?
 
he is doing short distance but I would imagine he should be getting at least 12mpg doing that. Not 10.

I have no idea about the seafoam. I guess I could try that.
 
Bad O2 sensor(s) maybe? I know when they take a shit, your MPG goes down the drain as well. It'll also run super rich and roast your cats.

Also a bad O2 sensor doesn't always set off a code.
 
I'm on 32's w 3.55 and 4.5 lift and my milage sucks too (~10 MPG on the speedo, uncorrected). 4:56 should help me out a bunch.

I've got a new engine and sensors, so yes, I think the gearing DOES make that much of a difference.
 
Just regeared today, I'll let you know.

Was getting 10.5 city. Started driving easy, got it up to 12.9.

Next step will be the O2 sensors.
 
33" on 3.55:1 (aw4) and still getting 12ish around town..... there's not a semi around that starts out slower than me, but once I get it going, I can cruise without trying to outrun or force my way into traffic-- I'm convinced it entirely how you drive that determines the efficiency of the engine (within reason, of course)
 
ty20404 said:
Nope, nothing... And as for the fact that it is reading under, it only reads about 3mph under
It reads 3mph under at what speed?
The speedo is a ratio - 3mph at 30mph = 6mph under at 60mph. It makes a huge difference to the calculation. You need to calculate the difference in terms of percent, not MPH.

Are the 31" tires new to the vehicle? What did you have before the change?
 
dunno if you have a computer near but if your o2 sensor is broken is usually will read lean (hince the poor gas mileage because its telling the car to dump fuel)

I think it was cheap if I remember.... helped me a little bit.
 
Was running stock 27" tires got 16.5mpg in town and 21mpg on highway went to 235/75/15 or 29" tires droped immediatly to 14 in town havent been on highway really yet stock 3.55gears. Decided to run around for a while with the tranny in POWER mode instead of Comfort and actually went up to 15.5mpg and the engine runs easier do to not shifting to early with the over all tall gearing with the new tires. Also when my O2 sensor took a dump I went from 21mpg highway to 15mpg highway and the exhaust smelled a little gassy at idle plus developed a little black ring around tip of exhaust from running in default mode...it sucked, put a new O2 sensor in and instant fix. Just my two cents worth.
By the way when the O2 goes bad, shorts, or what ever the ECM goes into a default rich mode incase you are at high altitude or wherever you wont blow an engine from running to lean, just remember to put new plugs in when you change the O2 sensor because they will probably be carboned up. yep more $

1989 XJ, 4.0L Renix, AW-4, NP231, D30/D35 3.55:1 for now, 3" Basterdized Lift
 
Last edited:
By a 4cyl commuter car....

That way you can still go to the parts store when they gave you the wrong part or go to work on Monday when the Jeep is trashed from weekend warrior-ing.

Jeff
 
I'm getting about 16-18 mpg with 31's and 3.55's. Since I have not corrected the speedo I calibrated the ODO with highway mile marker signs. 1 mile on the highway equals 9/10's of a mile on my ODO. So I figure my ODO is off by 10%. My ODO says I've traveled 250 miles, I've actually driven 275 miles.

When I fill up (after getting the loan approved) I always add 10% to my ODO before dividing the mileage.
Example:
250 miles/16 gallons= 15.6 MPG
10% adjustment
275 miles/16 gallons= 17.1 MPG.
 
ParadiseXJ said:
I'm getting about 16-18 mpg with 31's and 3.55's. Since I have not corrected the speedo I calibrated the ODO with highway mile marker signs. 1 mile on the highway equals 9/10's of a mile on my ODO. So I figure my ODO is off by 10%. My ODO says I've traveled 250 miles, I've actually driven 275 miles.

When I fill up (after getting the loan approved) I always add 10% to my ODO before dividing the mileage.
Example:
250 miles/16 gallons= 15.6 MPG
10% adjustment
275 miles/16 gallons= 17.1 MPG.

OMG! Get the speedo gear. It takes 10 minutes to change, 15 if you stop to take a P*SS! :twak: :laugh3:
 
i'm running 32's with stock gears...gas mileage sucks too.....was considering a warn front hub conversion so as not to push the front end all the time....figured i'd definitely get an increase in gas mileage...and with the cost of gas the increased mileage would save me $$ essentially paying for the conversion....does anyone have any numbers on increaded mileage with these conversions?? maybe a new post is in order here...
 
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