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Tires, suspension, MPG- decisions decisions

sobarr02

NAXJA Forum User
Location
California
Bought an Fed auction forestry '96 XJ 4.0. 5 spd a year ago after many years of lusting (in my heart only). All stock. 2 great surprises: On replacing all the gear oil found the limited slip in rear diff( shows how little I know) and am also getting up to near 25mpg hiway only ,with 235-75Rs ( at 28.9 about 1 inch larger dia. than stock). This at 4,200ft + elevation and prob no speedo adjust either. After extensive searching this forum this may sound incredible. One thing I found here, the MPGs are all over the map! The point is-a front shock is clunking badly and the ride makes a buck board buggy look plush. Enough reason to spend big bucks on this DD! Looking at mild lift quality suspension and going to 30-31' tires BUT: Priorities are 1. Nice ride on bad washboard and rutted dirt roads AND pavement! 2. Maintaining good MPG 3. Some towing, 4. A little more clearance, flex would be nice if not too costly (10% say) on MPG. I realize that the variables are infinite but any help apprreciated. I am now looking at "soft" OME kit and 30" max tires. All options on the table like aluminum rims. performance upgrades . Suggestions ?

Any one here that can say the impact of going from 28-29 to 30 " tire on MPG? Lastly- in theory larger tire should cover more ground per RPM and increase mpg except as heavier weight requires more engine work/gas than distance gained produces. Can steel to alum wheels save mpg with a bigger tire if not too fat? Any help appreciated TIA All
 
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The MPG difference between 29 and 30 inch tires won't be much. I have a 4.0 and AX-15 five speed, and am running 31s on my stock 3.07 gearing. I can get as much as 21 MPG highway. My highest MPG with 31s (20.87) is actually exactly the same as my highest MPG with stock 225s. I did switch from steel to aluminium wheels too, but I really doubt that that helped, especially considering that my al. wheels are an inch wider.

The Old Man Emu kit with 30 inch tires is a nice combination. I got to ride in an XJ with that combo in June and was amazed at how much nicer the ride was than my Rubicon Express 2" kit.
 
I have a '99 that I bought stock in great shape with low miles last year.
My purpose was to build an XJ that would get my canoe down most any trail to the river.
I went with Old Man Emu.. medium weight leaf packs in rear and ZJ coils in front.
ZJ coils are part number 934. Also used OME shocks all around and
OME steering stabilizer... Then I added JKS quicker disconnects and
JKS boomerang [dog leg] shackles on the rear springs.. Top it off with a set of
Bridgestone Dueler Revos 30x9.50x15 on stock rims and I now have a very capable
off road XJ, which also handles way better than stock when on-road.
 
P.S. to the MPG point,, the tires I replaced were Michelin AT'S 225-75-15
My mpg has gone up a little since the lift and tires installed..
With 4 speed auto I get about 16 mpg in city stop and go, and about
21 mpg on the 4 lane hiway with cruise set at 62 mph.
 
I try to stay off the interstates whenever possible.. too many
stupid'' no-drivin' idiots'' out there..... Also the old 4 lane divided U.S. and state hiways are much more interesting to me to travel on when I have to go the distance ....
Set your cruise on 55- 65 mph and I guarantee you will see better gas mileage...
 
Mambeu said:
The MPG difference between 29 and 30 inch tires won't be much. I have a 4.0 and AX-15 five speed, and am running 31s on my stock 3.07 gearing. I can get as much as 21 MPG highway. My highest MPG with 31s (20.87) is actually exactly the same as my highest MPG with stock 225s. I did switch from steel to aluminium wheels too, but I really doubt that that helped, especially considering that my al. wheels are an inch wider.

The Old Man Emu kit with 30 inch tires is a nice combination. I got to ride in an XJ with that combo in June and was amazed at how much nicer the ride was than my Rubicon Express 2" kit.

I suspect you're referring to my '92 here - if so, thanks for the kind word. I think it rides at least as well as stock (even with poly bushings in the leaf packs), and seemed to improve slightly when I went from 225s to 30s (OME actually recommends 31s for the 2 inch kit), which made me think that the system is designed with a certain unsprung weight in mind to balance the springs properly. It flexes real nice, and I haven't really noticed a major drop in MPG due to the lift/tires.

I'm disconnecting the rear swaybar tomorrow in preparation for a run to Paragon. Should be interesting to see if the ride improves again.

One word of caution - find a calculator on the web and double check your speedo - when I went from P225s to 30s, it threw my speedo off enough to make one mile on the road equal to 0.9 on the odo (with speed off by ~3-4 mph). Messed up my MPG calculations too. Based on charts I found online (such as GoJeep's site), I swapped the original 35-tooth speedo gear with a 33-tooth version, and I was properly calibrated again.

In case you're wondering about the difficulty rating on the speedo gear, I'd say opening and drinking a beer is harder. Plenty of good writeups exist online (such as GoJeep's).

I don't think you'll be disappointed if you go OME.

Rob
 
Mambeu said:
Yep. It was quite a shock to ride in your cushy 92 after driving 180 miles doorless in the pouring rain on mud tires and a stiff suspension. :D

Had a feeling. Sorry to hear you won't be making it down this weekend.

Did you ever get that tcase shifter fixed?
 
Re: Tires, suspension, MPG- Many thanks to Rick, Mambeu and Cushy Ride Rob ! You all

have helped me see the truth path to on road and off road spinal disc salvation. Again can't help but comment about the un predictablitity of MPG both stock and with some mods on these XJ. Mambeu vs. all the rest of you is a great example. Appears to me to be multifactorial, inaccirate odo meter, heavier wheels/tires, upping wind resistance, extra weight, and driving too fast (hear me Chili?). Both Slick Rick and Cushy Ride Rob have given me great set ups to consider and my Cherokee will be something like or duplicate of you guys, and I can only say i am really grateful for sharing your experience. Chili..... sorry to hear but I bet you got a ton o' mods and extra weight as well as heavy Left foot. But I bet you have a lot of fun in it too! It would be cool to see an article where common mods and MPG are tracked and accurately correlated. Later...
Little Grashopper
 
Re: Tires, suspension, MPG- Many thanks to Rick, Mambeu and Cushy Ride Rob ! You all

sobarr02 said:
driving too fast (hear me Chili?).
Chili..... sorry to hear but I bet you got a ton o' mods and extra weight as well as heavy Left foot. But I bet you have a lot of fun in it too! It would be cool to see an article where common mods and MPG are tracked and accurately correlated. Later...
Little Grashopper

I hear you....Actually I raised my tire pressure and I'm going to change my draving manners to see if my MPG's go up.
And i have a kinda heavy RIGHT foot. My left is normaly resting while driving..... :)
 
For the ride, towing, and off-road ability I would definetly go with OME, I did and love it.

As for fuel mileage, I regeared when I did my lift (3.55 to 4.10) and saw little or no change on the highway(20 mpg) but in town I saw a slight increase(15 went to 17mpg) and this was on the same set of tires before and after regear. The tires are 30" BFG AT.
 
Well, I have a bit of a different setup, but still could give a bit of insight. I am running a 3.5 in BDS system with 33x12.50s on aluminum wheels, all on stock gearing as of now (and probably will be for the next year, unless I find somehow to regear for a good chunk less than I have found so far). But all in all, I seem to be getting good mileage still. I have a 93 with the 4.0 and ax-15 5 speed, and while I expected no more than 15 mpg with the 33s (as opposed to 20+ avg per tank with combination highway/city driving with 30s) I am still getting just about the same mileage. Having the 5 speed is a HUGE plus when modifying and trying to maintain fuel economy I have found. Most people with the automatics complain about decreased fuel economy all the time, even just going to 31s. Most people with the 5 speed seem to maintain relatively decent fuel economy. I just filled up yesterday and put 100 miles on the Jeep (not adjusted for tire size, so I gotta multiply by a conservative 1.1) so close to 110 and I am just under 3/4 tank now. Not sure how well I am doing right now, but as of now an oil change is overdue and that helps alot.. but still.. good move getting the 5 speed. Just if you dont regear, dont be afraid to let the engine rev a bit. It likes to run in the high 2000s. I usually dont shift until 3000ish RPMs if not a bit higher in some cases. Just listen to and feel the engine, if it feels like its bogging a bit, downshift and let it rev up. I drive 70-75 (speedo reads 65-70) on the highway and I still get great economy, probably half of that time I spend in 4th gear. Hope that gives a bit of advice to someone.

Scott
 
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