I used to drive a Cherokee with stock tires and an auto transmission d30/ 8.25. Now driving a Cherokee with 31s and the same drive setup but the power is terrible. The engine has less miles and runs smoother but the power is way worse. I'm cruising at about 1900 rpm on the highway. Most say it is fine with 31s but that is not the case with mine. Do 3.55s have that low of rpm on the highway with 31s? I'm thinking of getting 4.10 s or 3.73 s. I drive mostly on road but do occasional wheeling.

 

your issue is somewhere else, i've been running on 31's for years on 3.55 gears along with many of my buddies on their other DD rides. I've never had an issue with power with 31's and 3.55's

 

unless your swapping whole axles out with those gears already in them then its not worth it to buy all new gears and install kits and all that jazz, just my opinion.

 

+1 i have 31's and 3:55's and have plenty of power...sounds like u might have another problem

 

I would agree, i ran 31's and 3.55's and never once thought i had a lack of power. but i also never have owned a stock one to base it off of. 32's even felt fine. 33's on the other hand started making freeway on ramps be depressing lol.

It is a lot of money to re-gear axles, that small of a difference wouldn't be worth the money in my opinion either.

 

fuel pressure weak air filter ? Somethings not right. 31 and 3.55 is plenty of power for me

 

Stock gearing in my jeep with 33's and it still has PLENTY of power, I can roast them from a stop and do 75 with ease.

 

You do realize that your speedo is reading 5-7 MPH slow with 31" Tire. If it says 60 you are traveling 65-67 MPH. If you at 1900 and 55MPH that is perfect, 65 on the speedo (70 actual) for me is 2400RPM. I don't drive 55 so IDK the RPM.

 

It has a new air filter already. Up any sort of incline on the highwayI need to downshift to third and if I put it to the floor I need to pray I can pass someone on a hill. I already am aware the speedometer is off. At 70 accounting for the speed I'm at around 2k. Also has neon objectors and a fresh oil change, brakes aren't sticking. Hmm

 

is it a renix or an H.O.?

maybe replace your plugs and wires. or at least gap your plugs to .035

i am running stock gears (3.55) on 36's with my auto renix.
sure, i'm not feeling like a ricer, but i have enough power as to not complain, really. i can still be the first off the light.
but it's relatively flat here, no mountains.

i just drive it like grandpa on a sunday morning.

 

It's an h.o. with msd coil new platinum plugs and wires

 

i make a trip to the upper Peninsula 2 times a year... everytime i make the trip i come accross 2 speed trap zones each way (with the speed gun and the public posted mph gauge) everytime i pass them i'm going 60 mph..... and everytime i look at my speedometer it says....... 60mph..... i've also ran my jeep next to many stock cars that my friends happen to be driving at the time.... 04 impala, 94 escort, 98 silverado, 95 stock XJ, 94 Geo prizm...... and everytime i've asked them how fast we're going... my speedometer has matched our actual speed.... i've even gotten a speeding ticket for going 70 in a 55, and what did my speedometer say...... 70 mph


so to finish up my rant, i think your exaggerating a little bit... Like a bunch of people seem to do.... unless all my friends cars and 2 interstate radar guns and a state police officers radar gun are wrong, running 31's on 3.55's will NOT throw off your speedo that much... a mile or 2 Maybe but deffinitely not 5-7

 

Yeah. Lots of variables here. I too have 33's and 3.55 gearing. But since we're on gearing topic. I would do 4.10's with your situation. Gives you wiggle room to move up to 32 or 33's. Get better gas and power.
However. I don't think it's gears. Check fuel system. Fuel pump, filter, get injectors cleaned. Check air filter. Do the cheap stuff first.
Also check diff fluid. If it hasn't been changed in a long while. It might be gettin thick. Creating some drag. That might be a little extreme of a thought.
But again. Do all the cheap stuff first. Check fluids/oils and fuel.
Your not getting any sputtering or hesitation while on the gas pedal are you?
Some times fuel line can be pitched.

 

Unless something is wrong with your speedo or it just magically adjusts itself it should change. With a tire thats any bigger the speedo is gonna be off. look here http://www.dakota-truck.net/TIRECALC/tirecalc.html you can type in the stock tire size and the tire you have and it shows how far off your speedo SHOULD be. Just type in stock as tire A and the size you have now as tire B. Then say compare tire A to tire B and itll show you

 

I have 31" coopers (30.4 actual), 3.55 gears, and an AW4, and when my speedo says 60 I'm going 63! It's off 5% at all speeds (verified by both my gps's). Granted all 31" tires are a different size though...