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Discussion Starter · #1 ·
Hey,

I have a '99 TJ 2.5L w/ stock 3.73 gears and an auto tranny. I recently upgraded to a 2" ProComp Lift w/ 31" Goodyear Duratecs (pic below :) ), but now I think my RPMs are much higher than that amount of lift/tire size warrants. I actually top out at 60 mph at 3100 RPM on a flat road, pedal pegged to the ground.

I guess I should go back and say that before the lift and tires, I was at ~70 mph at 3000 RPM.

Is this normal, or should I be looking for a serious power sucker (like a potentially clogged cat, or a faulty O2/MAF sensor...) Only had the Jeep about 6 months so far, so I haven't gotten the full hang of everything yet.
 

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With bigger tires you are probably actually going 70 mph did you regear or change the speedo gear.
 

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It is odd that you were at a lower rpm before yet you were going faster. With larger tires and stock gearing it should be lower rpms with higher speed. It may just be because of the auto tranny? I don't have one so I don't know much about them but im sure people with some good ideas will chime in shortly. Could it be slipping more from the stress? its probably just the fact that you need to regear to a lower gear, restoring your lost power. Your jeep might not be getting into its last gear because it can't with the higher gearing on larger tires. This would mean higher rpms while still a low speed like you say. Did you change the speedo gear with the bigger tires?
 

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31's shouldn't hurt it that bad but they will suck power, I'd regear. ive got 32"x11.5 on my 97 2.5 and the power is gone, im regearing the diffs to 4.88 ASAP though with the auto the ratio might be different than with my 5 speed. the lift is also hurting you, your getting more drag cause your higher up.
 

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Discussion Starter · #6 ·
It is odd that you were at a lower rpm before yet you were going faster. With larger tires and stock gearing it should be lower rpms with higher speed. It may just be because of the auto tranny? I don't have one so I don't know much about them but im sure people with some good ideas will chime in shortly. Could it be slipping more from the stress? its probably just the fact that you need to regear to a lower gear, restoring your lost power. Your jeep might not be getting into its last gear because it can't with the higher gearing on larger tires. This would mean higher rpms while still a low speed like you say. Did you change the speedo gear with the bigger tires?
You are probably right in that it isn't reaching the last gear. Guess the 2.5L/auto tranny just can't handle the bigger tires?
 

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Simcraft said:
You are probably right in that it isn't reaching the last gear. Guess the 2.5L/auto tranny just can't handle the bigger tires?
It should be able to take it, but you will probably need to regear to restore power and that last gear if this is really your problem. Maybe 4.56? Im not sure on what is best for your situation. Anyone know?
 

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Discussion Starter · #8 ·
31's shouldn't hurt it that bad but they will suck power, I'd regear. ive got 32"x11.5 on my 97 2.5 and the power is gone, im regearing the diffs to 4.88 ASAP though with the auto the ratio might be different than with my 5 speed.
I figured it shouldn't have hurt it that bad so I didn't even second guess the impact of it...

But, as for your current ratio, if you check the diff cover, it'll have the gear ratio on a tag on the left side (looking forward). Pretty sure early TJ's came stock with either 3.07 or 3.73.
 

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Yeah im pretty sure you will need 4.56 or 4.88, but not sure :thumb:
 

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Discussion Starter · #10 ·
Thank you for all your input guys. Looks like I'm going to save up some $$ to regear.

Or maybe just do an engine swap :whistling:
 

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Simcraft said:
Thank you for all your input guys. Looks like I'm going to save up some $$ to regear.

Or maybe just do an engine swap :whistling:
whole lot less work, and whole lot less money to regear.
 
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