PDA

View Full Version : Gear Ratio Change



elmer phudd
April 1st, 2012, 11:58 AM
04 TJ/LJ Stock 4 Ltr 6cyl. Cruise control, Stock Slush Box with overdrive, Rubi Crawler, Rough Country 4" long arm, G2 D44s front & rear geared 4:10, ProComp Extreme M/T 285/75R16 that measure @ 32" new. When I changed from 3:73 to 4:10 and 32's my speedo was right on checked with two GPS's. Thought that's cool I should be the same as stock. But in overdrive @ 65 MPH my Tach is @ 1850-1900 RPM and in Drive it's @ 2800 RPM. At 65MPH & 1900 RPM it's a dog, wont stay in overdrive on any rise in the road even a very slight long straight elevation change. And any overpass it falls on it's face and down shifts. I get out west and it will go right past drive to second and close to redline chaseing to get back to 65MPH. I drag a small trailer on dedicated trail runs 4'X6' to carry my offroad tires and steel rims, door dolly, jack & stands. I might as well be draging an anchor. The poor trany goes through fits trying to stay @ 65. I'm going to 33" drive tires with maybe a set of 35" dedicated offroad tires. Looking @ 4:56 or 4:88 gears. But I don't know how to do the math or formula to use to get an ideal of what gearing I realy should be @ to be in a power range of 2200-2400 RPM @ 65/70 MPH in overdrive on the highway . I might be wrong about that RPM range for best power MPG trade off . Not worried about offroad the Rubi Crawl takes care of the low end .
Any and all HELP would be Greatly Appreciated.
If there's a Gear Guru out there HEEEELP
Thank You
Elmer Phudd

phittie1100
April 1st, 2012, 04:09 PM
I spend 99% of my time on the freeway with the OD off - I am a step behind you, running smallish 33" ATs but still geared 3.73. But locking out the OD on a flat road at a steady pace allows the torque converter to lock up, reducing the rpms by about 250 or so. At 70mph, that leaves me turning about 2700-2800 rpms, 63mph or so works out to 2500 rpms which is where I spend most of my time driving. It actually gets better milage at either 2500 or 2800 than it does at 1900 lugging along trying to keep the torque converter locked up in OD. Also, because it is making good power at 2500 it only downshifts on steep hills with my boat or camper in tow.

I did the math, 4.10 should get me back to stock gearing but frankly it was a dog with stock gears and tires. 4.56 should get it done with 33s, but you should probably look at 4.88s if you are considering 35s. Until then, try switching off the OD.

elmer phudd
April 6th, 2012, 10:20 AM
Phittie1100
Thanks for the insite makes sense. If I regear I'm putting myself right were I'm already @. In drive @ 65 MPH 2750 RPM a little high and more fuel, but pulling a small trailer I could set the cruise and forget it. If I go to 35's with 4.10 I should be @ 2500 RPM @ 65 MPH power to fuel burn not bad and cruise should keep it there. I'd loose a little on the low end crawling, but the Rubi Crawler will take care of that.
Thanks Again
Elmer Phudd