thejim42 Posted August 4, 2014 Share Posted August 4, 2014 I have an 88 swb, 4.0 5speed, 2wd and currently have 235/75r15s and 3.07 gears. I'm planning on converting to 4x4 and wondering what gears I should use. I don't plan on going any bigger than 31x10.5s and I don't want to lose my 17.5 city mpg. Its my DD and I do both city and highway driving and plan on doing some light offroading. I want to pull a bass boat. Link to comment Share on other sites More sharing options...
Bill H Posted August 4, 2014 Share Posted August 4, 2014 Wow, Sounds like you're right where I was once. I have an 87 that I bought new as 2wd with 3.07s. I converted it to 4wd with an 88 mj front clip that had a dana 30 with 3.07s. Swapped easily. Then I ran up to 31 x 10.5 and it was a dog. I just installed 3.73 GS gears with solid single piece axles. It's like I have a different truck. Just my input, I hope it helps. Link to comment Share on other sites More sharing options...
Eagle Posted August 4, 2014 Share Posted August 4, 2014 I have an 88 swb, 4.0 5speed, 2wd and currently have 235/75r15s and 3.07 gears. I'm planning on converting to 4x4 and wondering what gears I should use. I don't plan on going any bigger than 31x10.5s and I don't want to lose my 17.5 city mpg. Its my DD and I do both city and highway driving and plan on doing some light offroading. I want to pull a bass boat. My rule of thumb is 3.73s for 30" tires, 4.10s for 31" tires, 4.56s for 32" tires, and 4.88s for anything larger than 32". For reference, I ran my '88 SWB with 31s and 3.73 gears. The overall final drive ratio (as determined by road speed per 1000 RPM) was exactly the same as my wife's bone stock XJ 4.0L with automatic. Link to comment Share on other sites More sharing options...
FrankTheDog Posted August 4, 2014 Share Posted August 4, 2014 3.73's or 4.10's if you go to a 32" tire. Rubicons come stock with 4.10 gears and 31" tires so they'll work just fine. You can always find a 4 cylinder TJ front end that has 4.10's in it. That's what I'm running in my 92 MJ. TJ Dana 30 4.10 front. MJ 4.10 Dana 35 rear. It has 265/70/16's on it and it's really peppy. Link to comment Share on other sites More sharing options...
thejim42 Posted August 4, 2014 Author Share Posted August 4, 2014 I have a 4x4 4cyl mj that I'm going to my 2wd for my little brother for His first car with 4.10s that I can use. What is that going to do for my gas mileage? Link to comment Share on other sites More sharing options...
Eagle Posted August 5, 2014 Share Posted August 5, 2014 With 4.10s and 31 inch tires your city mileage will probably be better than what you get now. Highway mileage will depend on how heavy your foot is. My '88 with 31s and 3.73 gears got 19 MPG highway. Link to comment Share on other sites More sharing options...
thejim42 Posted August 5, 2014 Author Share Posted August 5, 2014 I usuwlly drive 75...I'm scared to drive it any faster Link to comment Share on other sites More sharing options...
Pete M Posted August 5, 2014 Share Posted August 5, 2014 not sure if you're swapping just the gears or whole axles, doing the work yourself or paying a shop, but I figured I should add: do not spend any money on a Dana 35. swap in a whole (and better if you can) axle. :yes: Link to comment Share on other sites More sharing options...
thejim42 Posted August 5, 2014 Author Share Posted August 5, 2014 Swapping in a whole d35 out of my other truck Link to comment Share on other sites More sharing options...
COMJNUT Posted August 5, 2014 Share Posted August 5, 2014 with 31's get 3.73's, and get the lightest wheel/tire combo that you can find. Extra unsprung weight puts a major knock in gas mileage. Link to comment Share on other sites More sharing options...
Jeep Driver Posted August 5, 2014 Share Posted August 5, 2014 Swapping in a whole d35 out of my other truck Maybe we're all a little slow tonight..........? So, you are asking what gear to use? Yet, apparently, you state that you are swapping a D35 from a 2.5 MJ....correct? Doesn't that automatically answer the question? Or are you contemplating changing the gears out in the D35? If so, don't waste your time and money and effort. If you are keeping the 4.11s that come in the D35 then you will need a D30 from a 2.5 XJ or MJ with XJ being the likely donor as there are more of them. Look for the 94 up axle. I just took my D35 to the scrap yard and I got $18 for it.....that's what it's worth. Link to comment Share on other sites More sharing options...
thejim42 Posted August 5, 2014 Author Share Posted August 5, 2014 I'm debating on either swapping the axles out of my 86 mj which have 4.10 or 4.11 something like that. Keeping what I have or going with something else Link to comment Share on other sites More sharing options...
Jeep Driver Posted August 5, 2014 Share Posted August 5, 2014 I would not move a CAD axle. But it were a matter of rolling or not rolling.......... I just swapped over to the 8.25.........4.10 Took a couple months to find it, took a month or so to prep it and locate disc brakes etc........ But I take my time and do my best to go through it and get it right, others just throw stuff together. I would not hesitate with a 4.0 to go with 3.55s......any late model XJ with low miles from the JY.....round here front and rears are $150 and a dime a dozen. And think ahead, you'll eventually want the disc conversion. Image Not Found Link to comment Share on other sites More sharing options...
thejim42 Posted August 5, 2014 Author Share Posted August 5, 2014 Someone is going to have to school me on all the different axles. cAD, non-CAD, High and low pinion, open carrier, locked, I'm new to axles. All I've done is change gear oil, tomorrow I change axle seals in dads f150 but I've not done anything outside of that. I'd like to do everything myself because #1 at 18 money is rare, #2 I like learning new things and #3 it gives me a sense of accomplishment Link to comment Share on other sites More sharing options...
Jeep Driver Posted August 5, 2014 Share Posted August 5, 2014 Someone is going to have to school me on all the different axles. cAD, non-CAD, High and low pinion, open carrier, locked, I'm new to axles. All I've done is change gear oil, tomorrow I change axle seals in dads f150 but I've not done anything outside of that. I'd like to do everything myself because #1 at 18 money is rare, #2 I like learning new things and #3 it gives me a sense of accomplishment OK, :) Move both axles over.......you'll learn in time and new parts will come when the money comes. Link to comment Share on other sites More sharing options...
thejim42 Posted August 5, 2014 Author Share Posted August 5, 2014 Is a CAD axle one with the vacuum engage off the t-case to the passenger side? Link to comment Share on other sites More sharing options...
Jeep Driver Posted August 5, 2014 Share Posted August 5, 2014 Is a CAD axle one with the vacuum engage off the t-case to the passenger side? Correct and that side is a two piece axle. Some here like'm and say they make a stronger axle. But most are trying to lock them because they do not work. For myself........ I like everything to be clean and purposeful and working and the CAD serves no purpose IMO. Link to comment Share on other sites More sharing options...
Incommando Posted August 5, 2014 Share Posted August 5, 2014 Although maybe not optimal my '91 with a 4.0/aw4/231 combo on 32" tires runs just fine with the 3.55's. It is very livable on and off road. if you compare the RPM changes in each gear from a stock tire size & drivetrain combo to mine your average cruising RPM only changes 150 rpm for cruising speeds you see in daily driving. Yes. 150 RPMS. Stock RPM at 35 MPH = 1080 & 70 MPH = 2159. With the 32's then 35 mph = 978 & 70 MPH = 1957. To save you the math that is 98 & 188 RPMs respectively. The powerband of the 4.0 is just not sensitive or "peaky" enough to make those changes an issue. Anyone who claims that there is a major difference needs to quit listening to internet rumors and conventional wisdom ( you know---those tall tales that start out "well everybody says...") and actually research the stuff. You would probably effect your performance/MPG more by having a full tank of gas and a 250# friend in the passenger seat then by the tire size swap I referenced. BTW: the crawl ratio changes right at 1 point.... 26.94 to 25.88. As it is a trail toy and the on-road characteristics are less important to me I plan a swap to 4.56's in. But that is to optimize the jeep not for a specific purpose not because it is an issue as it sits now. Link to comment Share on other sites More sharing options...
thejim42 Posted August 5, 2014 Author Share Posted August 5, 2014 So there really isn't a noticeable difference in power between gears? What if I kept the 3.07s and 235s? Link to comment Share on other sites More sharing options...
Incommando Posted August 5, 2014 Share Posted August 5, 2014 Go here: www.grimmjeeper.com Enter the set-up you have and the set-up you want. You can play with different combos. Check the RPM's for how you realistically intend to use your truck to see if the current combo would be acceptable or what would. Link to comment Share on other sites More sharing options...
Pete M Posted August 5, 2014 Share Posted August 5, 2014 So there really isn't a noticeable difference in power between gears? What if I kept the 3.07s and 235s? there's a world of difference when changing ratios. i went from 3.07s to 3.55s (30" tires) and the truck really pepped up and came alive! I'll never settle for 3.07s again. Link to comment Share on other sites More sharing options...
Eagle Posted August 5, 2014 Share Posted August 5, 2014 So there really isn't a noticeable difference in power between gears? What if I kept the 3.07s and 235s? 3.07s with 235/75-15 tires is very workable -- it's only one tire size larger than what came stock on most of the late XJs. (Early XJs, even with the 4.0L, mostly had 205/75s or 215/75s.) Where you will be wanting more gear is when you move up to 31" tires. Can you drive on 31s with 3.07 gears? Yes, you can. I currently have the 31s from my '88 MJ on my stock '88 XJ with 3.07s. I can drive it, but it's doggy, and I am pretty well convinced that it actually hurts my fuel economy. Link to comment Share on other sites More sharing options...
Jeep Driver Posted August 5, 2014 Share Posted August 5, 2014 So there really isn't a noticeable difference in power between gears? What if I kept the 3.07s and 235s? 3.07s with 235/75-15 tires is very workable -- it's only one tire size larger than what came stock on most of the late XJs. (Early XJs, even with the 4.0L, mostly had 205/75s or 215/75s.) Where you will be wanting more gear is when you move up to 31" tires. Can you drive on 31s with 3.07 gears? Yes, you can. I currently have the 31s from my '88 MJ on my stock '88 XJ with 3.07s. I can drive it, but it's doggy, and I am pretty well convinced that it actually hurts my fuel economy. I think he's trying to understand the relationship between tire size and gear ratios. Without doing the math........ 3.07 with 235s are the equivalent to 3.55 with 31s or 4.11s with 33s and so on. Link to comment Share on other sites More sharing options...
Eagle Posted August 6, 2014 Share Posted August 6, 2014 This may help: https://spreadsheets.google.com/pub?hl=en&hl=en&key=0AvuGamS7bzt-dHJxYy1QajhuVUFUbUVDUWdiZnVFbmc&output=html Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now