55mph and no more...

[dynamic098]

So I bought a 91 YJ with a 2.5 litre. It has 31's on it with a 2" lift. Well I took er out and all I could get was 55 mph out of her.

How do I check the gear ratio ? Are the 31's the cause of this ? How can I change the setup

[jeep95xj]

does it sputter when you try and go higher? Do you have good power or whats expected out of a 2.5 all the way through to 55 mph?

[dynamic098]

No, it runs well. It gets to 55 smooth, but just doesn't have any more. If I am on flat ground with no wind I can get 65 out of er. That was in fourth gear, when I shift into fifth it will actually decelerate. That is why I am wondering about the tires. I am fairly certain that it has factory gearing.

[sredish]

2.5L with 4.88s and 32s.... Can easily punch through 70; never had it over 75 (rare one time occurrence). With stock gears and 31s, I could see the power loss but still think you should see 60... you may not use 5th gear much or at all. The 2.5L likes to revved high and held at high rpms to maximize the power of the motor.

[sredish]

factory gearing is 4.10 behind the 2.5, just fyi. sounds like you have stock gearing. if you were to change gears, i'd say 4.88 and no less. You'll easily enjoy 5th gear with it and even if you stay with 31s, your power will be much much improved.

[93jeepyj]

you may want to check to see if the speedo gear has been changed to make the speedo acurate with the bigger tires, you may be going faster then your speedo is registering.

[MJMPC]

Have you done a tuneup ?

[Pedro_Slick]

you have 4.10 or 4.11 gearing I have the same except with a 4 in suspension lift and 33 and I get 65, but it starts the death wobble at 63. Never use 5th gear, thats a slow down gear.

[Ptr]

I have 31's, stock gearing (4.10) and i can run 75 easily on the highway.
sounds like you need a tune up badly.

[acat]

depends how tired your engine is. I had 31s and a 2.5 liter and 65 was about tops. Could never use 5th. I gave the engine a tune up and it didn't help. Even tried the sea foam miracle treatment. No miracle for me. In my opinion you are looking at regearing or a 4.0 swap, which is the way I am going. Maybe youll get lucky and find something simple is wrong with it.

[1998zj]

With my 33's and 4.88 gears mine would run 80 mph in 5th gear. I have only had it up to 70 with the 35's I have on there now.

[jonzjeepyj]

I have a 2.5 that has....
1) 4.0L TB and bored out intake
2) 19lb Bosch design III injectors
3) Both air restrictors removed
4) Removed restrictive exhaust muffler and
replaced with a high flow glasspack

I have the stock 2.5L gears of 4.11 and run
33x12.5x15's and have not changed my speedo
gear and can run 70-75 uncorrected speed at 3200-3400 Rpms.
It's at least 12Mph slow. Yes I dont have 5th gear but it has a
lot of pep for a 2.5.

So do a few low dollar upgrades and rock on!

4.0L TB = 35.00 of EBay shipped to my door
19lb Bosch injectors off EBay of 12.00 shipped to my door
Removed both air restrictors = Free
Replaced muffler with high flow Cherry-Bomb glasspack
19.00 at local parts store
40.00 to have local muffler shop weld it on.
_______________________________________________

Total___=___106.00

[EABiker]

As mentioned, your speedometer probably has the uncorrected gear, (for 31's), in it. That will show nearly 10% slower than actual. You should be able to hit 70mph with your setup after a good tune up!

[pasinbuy]

I cannot use 5th either so on the highway I stay in 4th. If I did more offroad I might think about regearing but for not I am ok with not using 5th.

Please fill out you profile.

[dynamic098]

This Jeep has 102,000 miles on the 2.5. I did a complete body/frame resto on her and this was her maiden voyage. The engine was tuned up when it was reinstalled. It runs very smooth and has a lot of power. It has new exhaust front to back. New brakes and components on front and rear. New gear oil in the housings. I have not changed the gear fluid in the tranny or transfer case yet.

How difficult is it to change the gearing to the 4.88's ? Can the gearing be changed in the same differential or do I need to replace the axles to change the gearing ? Is the gearing change for both front and rear axles ?