pug Posted January 22, 2016 Share Posted January 22, 2016 I have a 1990 2.5 ax5. 1 should I just get gears installed and rear main seal fixed $1500-$2000. 2 just install 4.0. Link to comment Share on other sites More sharing options...
Pete M Posted January 22, 2016 Share Posted January 22, 2016 what gears do you have now? size tire? can you pull a trans yourself? would you be doing the 4.0 swap yourself? Link to comment Share on other sites More sharing options...
pug Posted January 22, 2016 Author Share Posted January 22, 2016 4 10s not sure whats up front. No on transmission. Getting help on swap Link to comment Share on other sites More sharing options...
87Warrior Posted January 22, 2016 Share Posted January 22, 2016 $2k would be a bargain for a shop to do gears AND a rear main (on a 2.5). You have to drop the transmission to replace the rear main on a 2.5. I am a big believer in the 2.5. It is a great motor. And honestly, if you are not going to do a 4.0 swap yourself, stick with what the truck came with from the factory. Will make trouble shooting in the future much easier. Plus you wouldn't be relying on somebody else to do the swap correctly. Link to comment Share on other sites More sharing options...
Pete M Posted January 22, 2016 Share Posted January 22, 2016 what size tire? what are the plans for the truck? Link to comment Share on other sites More sharing options...
Eagle Posted January 22, 2016 Share Posted January 22, 2016 what size tire? what are the plans for the truck? These are critical questions. You can't go much deeper with gears. You have 4.10s -- if you go with Dana-Spicer gears, there's only one more step: 4.56. You can get 4.88s if you go with aftermarket gears. But a lot depends on what you want to do with the truck and what size tires you run (or plan to run). Link to comment Share on other sites More sharing options...
pug Posted January 22, 2016 Author Share Posted January 22, 2016 Tire size is 31 10.50 15. Not going any bigger daily driver and use to pull 6x12 trailer with zero turn and walk behind mower. Would 4 56 make that big of a difference pulling. also would 4 88 be to low for highway driving without trailer. Link to comment Share on other sites More sharing options...
NorthWoods Posted January 23, 2016 Share Posted January 23, 2016 4.10 should be perfect with 31's. Anything lowet then that you will really start affecting how many rpms you are turning at highway speed along with lower mpg because you'll be turning higher rpm to go the same speed. Maybe you could go to 4.56 but I wouldnt recommend it unless you don't care about highway speed and fuel consumption. Link to comment Share on other sites More sharing options...
Pete M Posted January 23, 2016 Share Posted January 23, 2016 he's got a 2.5L. they need a bit more gear than the 4.0 to make up for the missing torque. and even 4.88s won't be too much for highway and a 4banger. heck, the 2.5/Autos came with 4.56 gears and they have 27" tires from the factory. :D please don't sink that kind of money into a dana 35. at least get a late model XJ 8.25 or explorer 8.8 and invest your money into an axle that will last. :thumbsup: Link to comment Share on other sites More sharing options...
pug Posted January 23, 2016 Author Share Posted January 23, 2016 On the flats 70 mph all day long 18-19 mpg. Incline downshift to 4th gear. Pulling trailer is all with in 5 miles no hwy driving but some hills Link to comment Share on other sites More sharing options...
Pete M Posted January 23, 2016 Share Posted January 23, 2016 did you swap in the correct speedo gear for the tires? your mpgs might be higher than you think. Link to comment Share on other sites More sharing options...
pug Posted January 23, 2016 Author Share Posted January 23, 2016 Yes sir. I was thinking about 4 88s just don't want truck running high rpms and still want to be around 15-16 mpg Link to comment Share on other sites More sharing options...
Pete M Posted January 23, 2016 Share Posted January 23, 2016 your mpg should improve with the better ratio. :thumbsup: Link to comment Share on other sites More sharing options...
outlaw star Posted January 23, 2016 Share Posted January 23, 2016 28÷31×4.88= 4.40:1 Link to comment Share on other sites More sharing options...
87Warrior Posted January 23, 2016 Share Posted January 23, 2016 your mpg should improve with the better ratio. Not necessarily true. I drove my TJ for awhile with 35's and stock 4.10 gears. Regeared with 5.13's and noticed a substantial drop in hwy mpg yet a slight improvement in town. If you want to know about how the truck will run down the highway with 4.88's over the stock 4.10's, don't shift into 5th gear. While the motor will be spinning a little faster in 4th than it would with 4.88's in 5th, I think you'd be able gauge the driveabilty pretty well. Link to comment Share on other sites More sharing options...
NorthWoods Posted January 23, 2016 Share Posted January 23, 2016 Yes at low speed less throttle will need to be applied in order to accelerate. 4.88's will require a higher rpm to achieve higher speeds. With 33's behind a 4 banger 4.88s would be great but with 31's you are going to turn significantly higher rpm to do say 65-70 mph. Link to comment Share on other sites More sharing options...
Eagle Posted January 23, 2016 Share Posted January 23, 2016 Yes sir. I was thinking about 4 88s just don't want truck running high rpms and still want to be around 15-16 mpg Here's a link to what I consider to be the most accurate chart of tire sizes and engine speeds with different axle ratios. I'm biased -- it's my spreadsheet. It's more accurate than most on-line calculators because I used actual tire manufacturers' revolutions-per-mile data rather than just calculating tire diameter, which doesn't allow for sidewall deflection when the tire is loaded. https://docs.google.com/spreadsheets/d/1pHFuhGgj6dQDfzyfFJH5z7NCDLW2KX3ABQgAJr3lBvM/pub?hl=en&hl=en&output=html Let's assume that a 4-banger originally came with 4.10 gears and 205/75-15 tires. Well, we can't check that, because I didn't go that small, so we'll use a 215/75-15 as the stock tire. Find the column for 4.10 gears, and we can see that at 70 MPH in 5th gear the engine turns 2705 RPM. So now we want to see what comes close to that with 31" tires. Scroll down to the section for 31x10.50-15 tires. We find that with 4.56 gears, at 70 MPH the engine will be turning at 2713 RPM. That's a difference of just 8 RPM from stock. Not worth even worrying about. That's the difference between new tires and tires that are 50% worn. If you want to jump to 4.88 gears, then we see that at 70 MPH (in 5th gear) the engine would be turning at 2904 RPM. That's faster than stock, but not that terrible. Remember, the 2.5L and 4.0L engines are basically the same design. They both were developed from the original 232 c.i.d. engine that AMC introduced in 1964. Back in those days, cars didn't have overdrives -- they were mostly 3-speed manuals, or basic 3-speed automatics. I had a 1966 Rambler American back then. With stock tires and stock gearing, it turned exactly 3000 RPM at 72 MPH. And those engines typically ran for well over 200,000 miles. So getting your 2.5L up to 2900 RPM at 70 MPH isn't going to hurt it. Just keep your oil clean and fresh. Link to comment Share on other sites More sharing options...
Pete M Posted January 23, 2016 Share Posted January 23, 2016 Yes at low speed less throttle will need to be applied in order to accelerate. 4.88's will require a higher rpm to achieve higher speeds. With 33's behind a 4 banger 4.88s would be great but with 31's you are going to turn significantly higher rpm to do say 65-70 mph. still less than a bone stock 2.5/auto Link to comment Share on other sites More sharing options...
pug Posted January 24, 2016 Author Share Posted January 24, 2016 So what brand gears would you recommend. Rear is 8.8 front is a d 30. Link to comment Share on other sites More sharing options...
Eagle Posted January 24, 2016 Share Posted January 24, 2016 The rear is an 8.8? Did you change it, or was it done by a previous owner? That pretty well rules out worrying about using Dana-Spicer gears, at least in the rear. Link to comment Share on other sites More sharing options...
dustyinthedirt Posted January 24, 2016 Share Posted January 24, 2016 28÷31×4.88= 4.40:1 Outlaw Star can you explain what this means? Link to comment Share on other sites More sharing options...
pug Posted January 24, 2016 Author Share Posted January 24, 2016 They are factory 4 10-4 11. Link to comment Share on other sites More sharing options...
Eagle Posted January 24, 2016 Share Posted January 24, 2016 They are factory 4 10-4 11. ??? What is this a response to? 4.10 was a ratio that the factory used with the 2.5L, but the Jeep factory didn't use 8.8" rear axles. We're trying to help you here, but trying to get useful information out of you is impossible. We can't help you if you won't answer questions. Link to comment Share on other sites More sharing options...
pug Posted January 24, 2016 Author Share Posted January 24, 2016 Oops on 4 10-4 11 reply. What brand of gears should I buy for 1998 8.8 and 1998 d 30. Link to comment Share on other sites More sharing options...
Eagle Posted January 25, 2016 Share Posted January 25, 2016 Oops on 4 10-4 11 reply. What brand of gears should I buy for 1998 8.8 and 1998 d 30. Maybe I'm just old (no question there) and dense (probably little question there, either) but I'm still not certain about your rear axle. "8.8" refers to a Ford rear axle, not a Jeep axle. It's a common swap for MJs, so I'm not saying you don't have an 8.8 -- but I'd just like confirmation that you know you DON'T have a factory Jeep rear axle under there. And if you have a 1998 front axle, what's it out of? Is it high pinion or low pinion? Link to comment Share on other sites More sharing options...
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