2.5 vs. 4.0

[Cj-Fan]

all it does is mess up your calculations when figuring mileage if you forget to factor in the percentage change. thats why so many people think they are getting horrible gas mileage, they forget to factor in the percentage of change from the tires being different. instead of your tire turning 1000 times per mile, it now only turns 878 times (just using imaginative numbers) so your odometer is showing you only get 150 miles per tank instead of 200 so it looks like you are getting horrible mileage. in reality, you are probably getting 18 instead of 20, or something close like that

Absolutely, There are lots of gear calculators out there. Gearing is "extremely important" for engine performance, mileage, torque, etc. Speedo gears for those who have ECU / ECM (computers) they need the correct feedback, and most vehicles actually need reprogrammed for drive gear changes, not just the speed-o, as they are designed to understand the difference between t-case hi / lo, and final drive! I love seeing these guys who think they can just go down to Discount tire, buy a 33 or 35 when they had the stock 29 (whatever the P-metic is for that) and wonder why it shifts different, doesn't "go" like it used to, and as mentioned, I can't get the mileage I used to.....Heres how you can calculate the change % (rough):

new tire diameter / original tire diameter. If we were to use 29 inch tire with new tire of 33, (29/33=.8787878...) that would be only 87% of original factory efficiency (torque, MPG, RPM) or a 13% loss (probably a little bit more - maybe as much as 15-18% real world due to excess engine load) Same applies to MPG's. This doesn't even take into consideration the ECM miscalculation.
The real defining factor is to match tire size with gears to match the original factory designed RPM, shift point, and highway cruising speed / rpm of the transmission.

You can still run a 2.5 and still see the equivalent RW Torque, and similar MPG, but you will never match HP and overall highway speed...I guess to stay within the title and post, there is never a replacement for displacement, the 4.0 will smoke the 2.5 any day.

 

[quicksand97]

Well said and makes since. But one thing. My Lil 2.5L will really scoots down the highway pretty fast. Grant you it is not or never will be a 4.0L, but the power to wieght ratio along with all its current mods under the hood this thing is a blast to drive. Fast take off, top end 75 to 80 MPh, Holds 65 to 70 on head wind with a medium upgrade uphill, and on a straight strecth of highway if I cruise at 65 MPH at less than 1/4 of pedal down I have torque when I push the skinny pedal down and clmbs to 75- 80 pretty fast. Now that being said I am sure if I swapped out my little 31s to something bigger, or added heavy duty bumpers and such FORGETTA BOUT IT. Like a old VW Baja Bug gotta keep it light.

 

[Jpwheeling2000]

Hey guys the one that commented to me about changing my gears, I just recently changed my Ring & Pinion to 4.56, thats made a huge difference got back most of my stock drive the person who's commented on the benefits of having a Electric Fan, hit the mark right on, must be a Mechanic, he's right when you only gain about 3 or 5 Horses, thats about what I gained. I also just changed out my Spedo Gear, gotten back the correct speed now, don't know if it changed anything. This little Four has no Power and gets horrible gas milage the only place where this engine shines is off roading, because it's lighter and has some low end torque when it's in 4 low, your better off with the straight six, lots of low end torque and great on street horse power and better gas mileage.

 

[TheGhost45]

Told you about the gearing, simple swap, everything back. glad I was useful.

 

[wesrule]

The 2.5 l makes 120 hp

 

[MY95YJ]

I have a quick side note Everyone says 4.88 in the 2.5 but what about 4.88 in the 4.0.
The reason why im asking is i want to do an axle swap with my already regeared axles to a 4.0 because it is looking like my jeep may not recover this time

I have 4.88's and the 4.0, Lots of fun with the street meats (265'), but I still want to go lower for the woods and my 36's.

 

[quicksand97]

So on my 97 2.5L with 31s and factory 4.10 gears what is the best gear ratio for a daily driver and some part time offroad. Might bump up to 32s later. Right now I do have plenty power with all of my mods under hood. O one more question with my current set up and if I swap to 32s will I lose alot of power?

 

[johntl6]

did the fan swap and 4.0 tb.. still a slug

 

[johntl6]

after taking out that fan shroud the engine has so much air the fans not needed on highway..

 

[quicksand97]

Man I cant believe your rig still a slug, Do you have cold air intake or a catback? Not trying to brag but my Little rig gets it, she is quick off the line, and hits top speeds real fast. I can cruise at 65MPH at about 1/4 pedal and still have passing power, hits 75 MPH easy. I also have not even done a E-fan yet. You gotta have some hidden power in there somewhere?

 

[johntl6]

well the ol' girl is rocking 165000 miles.. it gets up to speed fine, but doesn't seem to like the hills, (to be expected) the torque is great and I think its an awesome trail motor, having less weight on the front end. no cold air intake. exhaust is next on my list. what muffler you rockin?

 

[quicksand97]

I use a magnaflow muffler. Here is also a cheap mod and it will make a difference. Cut the downward elbow at the end of your tail pipe in order to make it straight at the end. You might want to add a exaust tip that has a bit larger dia. Nothing to getto. Drop a flat panel k&N filter and cut out the whole front of you stock filter box. No worrys its hard to get water in there. I clean mean about twice a year. But you probably only need to clean it once a year. I do have many other mods but try these for starters.

 

[Poop3000]

no replacement for displacement...

 

[quicksand97]

O yeah you you you 4.0L guys, I can go faster, Just kidding? But remember that power to wieght ratio. My Lil rig is pretty light and fast but your right it will never be a 4.0L, just a fast 2.5L. Its all good and does well. Happy Wheeling.

 

[W00GYB00GY]

4.0l ftw...

 

[johntl6]

ok. not sure what the cut down on the tail pipe will do. but ill work something out.. always like tinkering..

 

[quicksand97]

The cutting of the downward elbow will help as far as free flow exhaust wise to some small percent, especailly on a 4 Banger. Every bit counts, believe me.

 

[frhrwa]

I had a '93 YJ with the 2.5L manual trans.. it ran fine except pulling steep hills for any speed, off road would go anywhere any other jeep would go, plenty of off road power,, BIG difference was I could pass the gas stations that they all had to stop at.. I drove it from Las Vegas to Spokane and averaged over 20mpg with it.. only slowed down on a few really steep inclines.. otherwise, I cruised along at 75mph.. had a completely rebuilt 4.0L to drop in, AX15, wiring harness, and everything I needed.. sold it and then sold the YJ, and bought a CJ-7 w/258 in it.. big difference in gas mileage for sure.. power is a little more on the freeway getting to places, but off road can't really see any difference, other than the CJ has a 4" lift which is 2" too high for me, get nose bleeds up in that altitude, would swap this 4" lift for a 2" or 2.5" in an instant.. and these springs have less than 50 miles on them.. bought from BackCountry 4WD in las Vegas to replace the 4" that was on it, thought that was the steering problem, $500 wrong call..