Gearing change with Automatic trans

[BridgeStreetBlues]

With a TF999 Transmission , and 2.72 gears, would changing to 3.31 matter?

-Reason I ask is; I now have a wide 30 front and a narrow 20 back with spacers and want to change the back one to a wide track. ...So I found a matching set of wide tracks with 3.31 gears vs, my current 2.72 gears, for a good price. I only run 33's and drive on the road mostly so I wasn't looking for low gears.

Thanks for any advice.

 

[Old Dog]

It will actually help. The motor won't be lugging so bad.

 

[BridgeStreetBlues]

Cool thanks. I knew that would at least help with the tires anyway I just did not know how if it mattered with the tranny.

 

[Saddle Tramp]

Try these tables. It should give you some idea of RPM, Speed etc.

http://www.ramblerman.com/blazer/geartabl.htm

http://www.ramblerman.com/blazer/gearcalc.htm

PS: An auto Transmission allows for taller (numerically lower number) gears due to torque converter slippage, around 10% or so.

 

[BusaDave9]

As the charts that Saddle Tramp linked to your ideal gearing should be between 4.1 and 4.56 for 33 inch tires.
Since you found axles with 3.31 that's a step in the right direction. It's a lot of work to regear them so I would just leave the gears at 3.31.

 

[BridgeStreetBlues]

OK so maybe I'll wait for the right gear ratio. If I get into the 4. + range, will the rpm's be screaming down the road? Like I said in first post, this rig is mostly a driver and I put it in shows.

 

[BusaDave9]

If I get into the 4. + range, will the rpm's be screaming down the road?

No. With 33 inch tires and 4.1 gears the RPM will be back to what the factory intended.
In the above chart shoot for the green to bring the rpm back to factory. Yellow will be tall gearing and red is low gearing.
The numbers in the middle indicate the rpm at 65 mph.

 

[BridgeStreetBlues]

Now I understand, thanks

 

[Saddle Tramp]

No. With 33 inch tires and 4.1 gears the RPM will be back to what the factory intended.
In the above chart shoot for the green to bring the rpm back to factory. Yellow will be tall gearing and red is low gearing.
The numbers in the middle indicate the rpm at 65 mph.

In my 1978 CJ5 I have 33” tall tires (275/85/15) with 4.56 pumpkins and I’m not happy as far as street driving is concerned; OK crawl ratio but it feels like I need another gear to shift up to when on the street and at 65 mph I’m screaming at 3K and change. My first jeep was a 1972 CJ5 , AMC 304 , T-15 , Dana 20 , 3.73 pumpkins, and from what I remember about 30” of tire. It was my daily driver and was comfortable to drive long distances; even drove it to Montana from western Oregon with no problems. Of course at that stage of my life bouncing down the freeway at 80 MPH was common place but the point is the AMC 304 had plenty of power for the 3.73 gears.

So even though Busa’s advice is not wrong, to me if this jeep is your daily driver I would go with a little taller (numerically lower) gears. In fact I am considering bumping up my gears then adding a TeraLo kit to the Transfer Case to make up for the taller gears to keep or improve the crawl ratio.

 

[micrablue]

BridgeStreetBlues, what did you decide to do?

I am currently gathering parts for my swap: manual to auto in my CJ7 .

There are many factors in deciding what stall speed I went with for my swap: camshaft specs, axle gearing, and tire size to name a few. Why do I bring this up? Because these are the known factors in my case, which directly decide what stall speed my torque converter will be happiest.

For you, your knowns are: engine peak torque at RPM (camshaft specs), 2.72s (axle gearing), and 33's (tire size). In your case, you also have the stall speed of your torque converter already, which also affects drive-ability in your case.

If you change one of these factors, it affects the other, which everyone knows. For manuals, if your engine lugs, you can downshift. Then either change gearing or change tire size.

For automatics, if your torque converter never locks up, and slips the whole time due to never reaching the speed it needs, it slips = heat = burns up. Also, by changing gears/tires, you may change where the sweet spot of where peak torque can occur.

Bottom line, you may need to do some homework on your case. I don't know enough about your particular Transmission and torque converter, but just want to make you aware of some factors from my own personal case and what I used to make my decision.

I will also add, with an automatic, in a Ford Superduty (diesel), you can opt to have the tow package, which comes with a lower gear ratio, and still keep the same powertrain as Ford offers as the standard for higher gear ratio they use for economy. This is also due to the diesel's low RPM high torque character. Using this same thought, with the Jeep's AMC 258 i6 / 4.2l low RPM high torque character, and your selection of a *close* gear ratio of 3.31 (vs 2.72), you should be OK!

 

[BridgeStreetBlues]

I just went with the 3:31's mainly because I could not find anything else without spending a ton. I only paid $300 for these axles and all I had to do was put inner seals in the front and a pinion seal in the back rear and paint them of course. I only drive my rig on the road and show it at times so for me the 300-500 RPM difference between these and say 3:73 or 4:11's did not matter. I can alwas re-gear later if I wanted, for now I have a nice set that had all new ball joints and bearings in the front and are both wide track. SO, for me this worked, it also put my speedometer back on the money. As for TC, I just used a new stock one. She runs fine to me, way way better than before with 2:72's even with a slight decrease in gears.