Big Wheels, Big Problems

I just helped my son put a Rubicon Express 4 1/2" Lift and a 1" Body Lift on so that he could put Goodyear 35X12.5 tires on his 2004 Rubicon. The engine RPM has dropped as expected to about 2000 rpm at 60MPH in 5th gear. Acceleration and highway hill climbing is very poor and shifting down to a lower gear, although it gives higher RPM does little for the power, just more noise.
Is this what's to be expected with larger tires? I know some of you have gone to bigger rubber, what problems did you encounter.
It could also be a problem of low engine power, a sickly engine will sure show up with bigger tires.
I can only compare it to my 94 Jimmy with 31" tires. The Rpms are about the same, but it has much more power, even with an auto and over 200K miles.
Any help will be appreciated.
Thanks
94

I have 33's on my Wrangler and I've noticed the same thing. The acceleration will suffer, the torque at the wheels will suffer... 35's are significantly larger than stock. You could try changing the gearing.

Larger diameter tires = higher gearing = less acceleration.

I just helped my son put a Rubicon Express 4 1/2" Lift and a 1" Body Lift on so that he could put Goodyear 35X12.5 tires on his 2004 Rubicon. The engine RPM has dropped as expected to about 2000 rpm at 60MPH in 5th gear. Acceleration and highway hill climbing is very poor and shifting down to a lower gear, although it gives higher RPM does little for the power, just more noise.
Is this what's to be expected with larger tires? I know some of you have gone to bigger rubber, what problems did you encounter.
It could also be a problem of low engine power, a sickly engine will sure show up with bigger tires.
I can only compare it to my 94 Jimmy with 31" tires. The Rpms are about the same, but it has much more power, even with an auto and over 200K miles.
Any help will be appreciated.
Thanks
94


Biger is not aways better.
Sometimes a little bit is better.:grinyes: :grinno: :lol:
Besides engine power bigger is a lot harder on brakes and front end.

The engine and computer system and tarnsmission and rear end and front end is designed and set up for the OEM sise tires.

You need to change rear end gear ratio to match the bigger tires or just downsise tires.
MT
MT

Its not just the increase in tire diameter that gets you, its also the mass or weight of the tire that steals power. Think about the engine trying to spin all that extra mass, further away from the center of the hubs, multiplied by four.
You can go with 4.88 gears to get your RPMs back where they should be but your engine is still trying to turn those big heavy tires. Not much you can do about the phyisics except get smaller, lighter tires or build up the engine. You could go with lower gears like 5.38s. That would help your power but now your RPMs are higher.

The rolling mass of those big tires also effects braking. Its as hard to stop that much mass from spinning as it is to start it.

I never would have thought that the size of the tire and wheel made much of a difference, but on our Rubicon, we use 215/75/15 in the winter. The brakes are much more responsive than they are with the 31'' MtRs

well, if you want to run big tires you have some options for better accellaration. change the rear diffrential, work the engine, decrease tire size or work engine, change gear ratio and tire size for MAX performance hehe

I am runnin 34's on my 05 Rubi, and didn't notice that much of a differance. I have the 6 speed also though. I am not sure what your lookin for in the acceleration department, mine has a tendancy of pickin up a tire if I dump the clutch at 3500.

if you change the rear gears , wouldn't you have to change the front as well? everyone if talking only about the rear.

Well, you don't have to, you could go to smaller tires up front to balance it out. Or just not use 4wd. I say this not to be a smart ass, but because I have seen it. I AM from Alabama.

i put 31x10.50's on my stock wrangler x, notice a bit of acceleration problems nothing too major, is it doing any harm to the car, if it is what can i do to fix it, the tires are not huge just enough to give it some better looks?

Yes big wheels can or will do a lot of harm.
Always check the date on a post. This old is old and dead.
It would be best to start a new thread/post on your problem.
MT