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Unread 07-15-2014, 07:28 AM   #1
spdracer86
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1998 ZJ 
 
Join Date: Jul 2014
Location: fayetteville, nc
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Delima, delima, delima

I have owned my 5.9 ZJ since it was new back in 1998. Just last week I finally made my first modification(s). Rubicon Expess 3.5 SuperFlex, RE Front Adj UCA's, Bilsteins, Tom Wood's front DS, Rubicon 17 inch wheels wrapped in BFG MT 255/75/17. I have NOT corrected the speedo. But I do know that when my speedo reads 60 mph, my gps shows i am doing 65 mph. This is a 8% difference.

Before these mods, I would average anywhere from 15.xx to 16.xx mpg. Now my travel computer states I am getting 10.2 to 10.4 mpg. Taking into account the 8% error rate i am guessing its actually closer to 11.0 to 11.2 mpg.

These MPGs are from just cruising around town and commuting to and from work.

The factory gears are 3.73's. Because of the larger tires I am assuming the extra fuel consumption comes from the extra effort it takes to get the vehicle up to speed.

So my question; If I were to change the gearing to 4.10s or 4.56s would this help to regain some of my lost fuel economy?

Any and all serious thoughts are welcomed.

Thanks

Bryan

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Unread 07-15-2014, 09:31 AM   #2
PTECH
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Interesting that your speedo shows lower than what you're actually doing. My 98 with 33s shows 80 mph on the dash and my gps shows 70 mph. Thats with 33s. I would suspect that regearing would actually cause you to loose milage since you would be running the engine at higher rpms but I'm not expert. I ***** foot mine around and Im getting 13.4 according to how many gallons I'm putting in the tank divided by the mileage from my gps.
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Unread 07-15-2014, 09:53 AM   #3
T-nave
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Gears might help a little but it would take forever for the you to see any kind of return on your investment. I have a 5.9 with 33s and average around 12-14mpg city. I went on a trip up north going an average of 60mph and got over 20mpg. Freeway is around 15mpg but I like going 75-80mph. I changed all my fluids, cleaned the throttle body, k&n cold air intake, upgraded the ING. and put an exhaust on it. You will be looking at over 700$ just in parts for gears. If you have somebody put them in for you then you will likely have 300-500 more into it. I wouldnt bother with it just for MPGs if I were you. You might not even get anymore out of it. However it will crawl around and take off from the line better.
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Unread 07-15-2014, 10:01 AM   #4
Timo_90xj
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Quote:
Originally Posted by spdracer86 View Post
..my first modification(s). Rubicon Expess 3.5 SuperFlex, RE Front Adj UCA's, Bilsteins, Tom Wood's front DS, Rubicon 17 inch wheels wrapped in BFG MT 255/75/17. I have NOT corrected the speedo. But I do know that when my speedo reads 60 mph, my gps shows i am doing 65 mph. This is a 8% difference.
Sounds about right. 255/75 R17 is pretty close to 32" diameter, with the 32s I had with stock gearing my speedometer showed very close to actual speed I was doing according to a GPS.

When you go higher on tire diameter without changing speedo gear or going to shorter gear ratio on the axles, you get closer to actual speed what your speedo is showing. Go large enough, and speedo is showing less than what you're driving.

Quote:
Originally Posted by PTECH View Post
Interesting that your speedo shows lower than what you're actually doing. My 98 with 33s shows 80 mph on the dash and my gps shows 70 mph. Thats with long arms and 33s.
Well that's kinda weird really and should not be physically possible, if you're running stock gears and stock speedo gear.


Quote:
Before these mods, I would average anywhere from 15.xx to 16.xx mpg. Now my travel computer states I am getting 10.2 to 10.4 mpg. Taking into account the 8% error rate i am guessing its actually closer to 11.0 to 11.2 mpg.
11mpg sounds a tad low, but that really depends how much stop-and-go traffic you're doing and is the area you live in hilly or pretty flat. You can get the MPG to drop to as low as around 11-12 doing city driving on stock gears and 32s. I was averaging 20-22 litres/ 100km with stock gears doing 50/50 city/highway driving (or even more on cold winter mornings), That calculates to around 10.5-11.5 mpg.


Quote:
The factory gears are 3.73's. Because of the larger tires I am assuming the extra fuel consumption comes from the extra effort it takes to get the vehicle up to speed. So my question; If I were to change the gearing to 4.10s or 4.56s would this help to regain some of my lost fuel economy?
I highly suggest doing so, although you most likely will not gain your money back from filling up less often within the near future - if ever.. Regearing is expensive. So it's not so much about the money, but more so because you will get your performance back to what it was, and you also reduce the risk of burning your tranny (niners do NOT have an auxiliary tranny cooler, which they should have!), and if/ when you go wheeling, it won't suck because of too tall gears on larger and heavier-than-stock tires.


I changed to 4.56 gears, and I'm now averaging ~16-17 mpg on the highways doing between 55-65mph, and ~13-15 mpg in purely city driving. I am also back to about stock performance; the heavier tires and other off-road mods have raised the weight of my Jeep by about 200lbs., so all that shows in both performance and gas consumption even though I have the shorter gears. edit: If you are looking more into reduced MPG than performance, you might want to look for 4.11 gears. Then again, if you ever go higher in tire size - which is more than likely, if you start taking your niner offroad - you will want shorter gears.
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1990 XJ Limited (4-door), 4.0 I6, AW4, NP242, ***rolled and totalled @ 165k miles***

***Under construction***
1990 XJ (4-door), 4.0 I6, AW4, NP242, PBR 42" tires, Unimog 404 portal axles, 110" WB, full cage + uniframe completely rebuilt, front 3-link + panhard / double triangulated 4-link rear,... ***SOLD***
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Unread 07-15-2014, 10:04 AM   #5
T-nave
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Everybody that owns a niner should put a tranny cooler in. NOW!
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Unread 07-15-2014, 10:06 AM   #6
Timo_90xj
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Quote:
Originally Posted by T-nave View Post
Gears might help a little but it would take forever for the you to see any kind of return on your investment. I have a 5.9 with 33s and average around 12-14mpg city. I went on a trip up north going an average of 60mph and got over 20mpg. Freeway is around 15mpg but I like going 75-80mph.
Are these numbers from the overhead console, or actual MPG ratings calculated using a GPS to track your path and full tank fill-up and then filling back up and doing number- crunching?

My overhead console showed excellent numbers with 32s and stock gears, but I wasn't even close those kind of MPGs even on flat ground doing 55-60mph if I calculated the actual mileage. I was only getting better MPG with stock gears if I was doing 75mph+


Quote:
Originally Posted by T-nave View Post
Everybody that owns a niner should put a tranny cooler in. NOW!
Absolutely true. I haven't installed mine yet (still sitting on the garage shelf ), but I do have a tranny temp gauge that shows the oil pan temps. It doesn't take much stop-and-go traffic in summer city traffic to get it up to over 175*F...
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1998 Grand Cherokee 5.9 LX daily driver, 1.75" BB, 32" KM2s, HPD30 Eaton e-locker/D44a stock LSD, 4.56 gears, custom- fabbed tube bumpers and tube fenders,...

http://www.jeepforum.com/forum/f13/d...s-etc-1222317/


1990 XJ Limited (4-door), 4.0 I6, AW4, NP242, ***rolled and totalled @ 165k miles***

***Under construction***
1990 XJ (4-door), 4.0 I6, AW4, NP242, PBR 42" tires, Unimog 404 portal axles, 110" WB, full cage + uniframe completely rebuilt, front 3-link + panhard / double triangulated 4-link rear,... ***SOLD***
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Unread 07-15-2014, 10:14 AM   #7
PolkaPower
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Put a trans cooler in right away. My speedo shows slower than I am actually going according to my GPS with skinny 31's. Even with those tires and a 2" lift I have never come close to actual 20 mpg unless I had my foot off the gas, was going downhill, and watching overhead.

I would do 4:56, 4:10 is a waste. Not much of a difference at all.
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Unread 07-15-2014, 10:34 AM   #8
PTECH
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Quote:
Originally Posted by Timo_90xj View Post
Well that's kinda weird really and should not be physically possible, if you're running stock gears and stock speedo gear.
Well I just bought it so I dont know if the gears or the speedo gear have been modified but thats what it does lol! There are no identification tags or anything on either of my diffs. Is there an easy way to tell what gears you have without tearing things apart and counting teeth?
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Unread 07-15-2014, 11:24 AM   #9
PolkaPower
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All V8 ZJ's came with 3:73 gears. Their are metal tags on the diff covers that tell you gear ratio and axle type. Other than that you would have to open it up and count.
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Unread 07-15-2014, 11:30 AM   #10
T-nave
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I don't know if I'm getting lucky or maybe it's b/c I'm not taking into account that the first half of the tank goes slower then the last half ( I'm not sure that last part is true). I have done the same trip twice now. I drive it to the blessing of the jeeps which is about 150 miles away and I use 1/3 of a tank. 23 gal / 3 ~7.5. 150 miles / 7.5=20. Even if my math is a little off I'm happy with it. This is on a road that I can only go 60-65 but it also has several little town to slow you down on the way.

To the OP. If most of your driving is around town then a higher numerical gear would probably help quite a bit.
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Unread 07-15-2014, 12:01 PM   #11
Timo_90xj
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You can't trust your fuel tank gauge; even the odometer will be a bit off with non-stock sized tires and the overhead computer isn't very accurate either. You'd have to drive a known verified route length with a full tank of gas, then refill at the end and calculate.

I find it almost impossible to believe a lifted ZJ niner on 33s would ever get 20mpg on anything but a long stretch of downhill. Even on a stock niner it's not easy to achieve 20mpg driving on highways or backcountry roads. In city, impossible.
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1998 Grand Cherokee 5.9 LX daily driver, 1.75" BB, 32" KM2s, HPD30 Eaton e-locker/D44a stock LSD, 4.56 gears, custom- fabbed tube bumpers and tube fenders,...

http://www.jeepforum.com/forum/f13/d...s-etc-1222317/


1990 XJ Limited (4-door), 4.0 I6, AW4, NP242, ***rolled and totalled @ 165k miles***

***Under construction***
1990 XJ (4-door), 4.0 I6, AW4, NP242, PBR 42" tires, Unimog 404 portal axles, 110" WB, full cage + uniframe completely rebuilt, front 3-link + panhard / double triangulated 4-link rear,... ***SOLD***
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Unread 07-15-2014, 12:14 PM   #12
PTECH
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Quote:
Originally Posted by Timo_90xj View Post
I find it almost impossible to believe a lifted ZJ niner on 33s would ever get 20mpg on anything but a long stretch of downhill. Even on a stock niner it's not easy to achieve 20mpg driving on highways or backcountry roads. In city, impossible.
Agreed. Estimates for a niner are 11/16 and thats using the old school EPA much more optimistic test method.
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Unread 07-15-2014, 12:35 PM   #13
T-nave
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Must be my gauge is off. I've run it till the gauge is down to the last notch and when I fill it it goes a lil past F? I'm happy either way with my niner. I'm the type of person that builds something then gets bored with it and moves on to the next project. I still haven't found a vehicle that isn't crazy expensive, meet my needs and be better then what I have now.
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Unread 07-15-2014, 01:36 PM   #14
PolkaPower
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There is still 2-4 gallons left in the tank when it hits the E mark. When I dropped my tank I drove it past the E mark and there was still plenty of fuel left for it to be a little cumbersome.
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Unread 07-15-2014, 02:00 PM   #15
Timo_90xj
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If I let my tang gauge drop a bit closer to 1/8th left, I may end up on the side of the road with an empty tank. Been there, done that - twice I've never seen my yellow fill up light illuminate, not even when I've been left on the side of the road. I've dropped the tank and checked the float system for the gauge, and it works. Weird things.. but this is a Jeep so I'm not wondering


I'm very happy with my niner, even with the amount of gas it burns and the cost of gas over here During warm season, I do prefer taking the metro and bus, or bicycle to work though.. better to pay 50$/ month for unlimited use of public transport, than 350$/ month on gas


As for the OPs questions, a lifted niner with 32s or 33s, I'd regear it. I didn't like my Jeep when I drove it on stock gears and 32" tires.
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1998 Grand Cherokee 5.9 LX daily driver, 1.75" BB, 32" KM2s, HPD30 Eaton e-locker/D44a stock LSD, 4.56 gears, custom- fabbed tube bumpers and tube fenders,...

http://www.jeepforum.com/forum/f13/d...s-etc-1222317/


1990 XJ Limited (4-door), 4.0 I6, AW4, NP242, ***rolled and totalled @ 165k miles***

***Under construction***
1990 XJ (4-door), 4.0 I6, AW4, NP242, PBR 42" tires, Unimog 404 portal axles, 110" WB, full cage + uniframe completely rebuilt, front 3-link + panhard / double triangulated 4-link rear,... ***SOLD***
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