Do I need to change diff ratio..

With 33's you may be ok with the 3.73's and you may want to run it that way for awhile to see how you like it. It will be very noticeable vs. 30" tires. The comparative jump to 4.10s or 4.56s will be very noticeable with 33s, in fact on my 94 YJ 4.10s and 33s were virtually perfect, plenty of reduction for off roading and a great cruising gear. It's got 4.88s and 35s now and probably can stand a jump to 37s before a re-gear.

Thanks Bill & Gerald,

I guess I'll stick with the ratio I have know and give up 5th like you say Bill. I don't particularly like it now at 2150 RPM's. Kind of weak in 5th. I see I will only have to go to 4.11 if I change the ratio. Fuel mileage should be OK and I can use 5th downhill. LOL

4.10's will keep your RPM at a given speed about where they are now. And you probably won't need to get a new speedo gear. I agree with Gerald, see how it performs with 3.73's. You might get slightly better fuel economy as well. Though when I went to 33's I changed to 4.56's, and seemed to increase my economy by 2-3mpg. Speedo gear is correct. Maybe it's because the engine is in a more efficient powerband, or that I rarely go above 65 mph now.

• * * Matt Macchiarolo
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Having run 33" tires and 4.10 gearing for several years, I'll say that 4.56 is definitely the optimal ratio to go with for 33" tires. :)

Jerry

Thanks Matt,

If I go to 4.10's I will probably still be able to use overdrive (5th). Like Bill related to, with my 3.73 I can forget 5th at 65. And I am like you, seldom over 65. Right now, in 4 low, second is too low for most trails. Third is better even up hills. Of course tires have a lot to do with this. I may just leave it at 3.73 and see how it goes.

Andy

I get the best mileage running my 86 at 2300 rpm, 19 US mpg.

If I use 5th, I drop well below 2000 rpm and then my gas mileage sucks.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Andy wrote:

Hi Mike,

Exactly my thoughts. Actually, my TJ with stock gearing is barely suitable for street use. My new thoughts are to change gearing soon after lifting and other mods.

I believe my 5th gear is 0.79 (Overdrive ratio). So at 65 and a 3.73 gear set, and 33" tires, I will turn 1975 RPM.s. So fifth gear is out as Bill pointed out. In fourth I will turn 2469 RPM's and that fine.

Now if I install 4.56 gears, in 5th I will turn 2414 RPM's. I like that and the fact that I will have all 5 gears available.

I'm still looking into this but so far this looks the best. 4.10's will give me 2171 RPM in fifth. A little low.

With 30" stock tires I turn a little under 2200 RPM's in 5th. Not much power there. At 2414 RPM I might like it and have 33" tires to boot. And that's at 65 MPH.

This is fun!

Andy

Another thing you have not mentioned is that 1st gear with the stock gearing is way too tall, meaning that slow speed travel is difficult to impossible without slipping the clutch. Going with 4.56s will let 1st gear work at low speeds again.

I am turning 33's with 3.31 gears....

5th at 65 is more like 1800 rpm for me. 2300 rpm gets me close to 70 mph.

As an experiment, I ran my Cherokee with P235's on it in 4th only on our last trip into the bush. The last 24 hour gas station is exactly 200 highway km from my local fill and the Cherokee got exactly the same mileage without having to shift into 5th and back down to 4th if I wanted to pass or something.

It was easier to drive too. I only need a twitch on the gas pedal to adjust for wind or a hill where in 5th, it takes the full throw to the floor sometimes, then you have to think on down shifting, etc... More work.

My odometers and speedometers are right on with each other and my GPS. Well, my Cherokee is almost 0.1 km different on that 200 km run, we do it often.

I have no issues in town. 3rd is good up to a 40 mph cruise for me and keeps me in the 2300 range and the 'no speeding ticket' range where I get decent city mileage. 2nd will haul out to 50+ mph at 4500 rpm if I really want to scoot... ;-)

Same for off road, I seldom need or use 1st low, I drive mostly in 3rd low, 2nd down hills.

Mike

Andy wrote:

Right!

I miss my 79 CJ7 with the four speed, T4 (I think) tranny. That would crawl.

So I guess I will shoot for a 4.56 ring and pinion. That should give me a good 5th and good gearing for off road in 4low.

Now all I need is cash! Thanks to all for assistance on this project. I want to do it once, correctly.

Andy

DETAILS! they will get you every time.