4x4 Gear swap -> 2.73 Rear

[rudypoochris]

I have a 1992 4x4 with 3.27 gears. I want to go to 2.73's in the rear, but don't know what to do with the D35 front. How close do I need to be ratio wise for it to be even "acceptable" in snow or gravel use? Does anyone know if I can find a ratio close to 2.73 for the D35 front. If I do do the 2.73's in the rear, am I basically going to have to kiss 4x4 good bye?

 

[carlover]

2.73????????? kiss your trans goodbye with that high of a ratio.

 

[rudypoochris]

2.73????????? kiss your trans goodbye with that high of a ratio.

Please do explain? I am not using this car to "4x4" as in crawl and what not. I am using it on the highway and such and occasionally go to the snow and would like to be able to use 4x4 if I need to get out of a ditch or something similar. Nothing more.

Also, could you provide insight to the other questions please. Thanks!

 

[carlover]

with that high of a gear ratio your trans will be struggling and way out of the powerband of your motor. for gear ratios think the higher the number the lower the ratio and lower the number the higher the ratio. Id say with stock sized tires you want something with 3.73's not 2.73's. And if you regear the rear you must regear the front to keep 4wd.

 

[rudypoochris]

I want it to be cruising 1630 or so RPM at 65mph. I think the trans will be able to take it just fine. Thanks for the advice though! I know it will be a bit of a dog as well. I want the 2.73's for economy. Any insight on D35 availability in that range?

 

[Burns]

I dont see this being a good idea at all.

I agree that it will strain everything. Engine and tranny.

 

[trevell407]

why 1630 of all numbers?
I think with that ratio your fuel economy will be less that now unless you're always doing 65 mph.

 

[rudypoochris]

why 1630 of all numbers?
I think with that ratio your fuel economy will be less that now unless you're always doing 65 mph.

Because 1630 is approximately the rpm that a 2.73 will turn at 65mph. That is the RPM I want it to turn. It will not be worse fuel economy. The long gear makes the engine work harder, yes, but at a lower RPM. This causes the throttle body to open wider which reduces pumping loses. I know for sure that running this rpm instead of the 1960 or so I am doing now will net me better economy and engine longevity. It may strain the tailshaft bearing or whatever of the transmission, but I highly doubt it. Also, remember these cars were setup for 55mph speed limits and cruising. At 55mph the engine turns 1655rpm or so, which incidentally is great for fuel economy compared to the 1960 or so I am turning now at 65mph... normal cruise speed is closer to 72mph though, which is why I want to run steeper gears.

In any case, does anyone know where to source D35 gears in about 2.73? If I can't find them, I will probably talk to a gear shop, then from there probably just run the 2.73 rear and keep the stock front or remove the 4x4 axles, gear, front DS etc. We will see. I guess 3.2x on snow with 2.73 rear probably isn't that bad... on snow...

EDIT: Silly me, I don't know for sure. I am 99% sure though.

 

[Burns]

my 4.0 5 speed explorer with 4.88 gears and 36s is getting 17.5 every tank.

I run 2800 at 65


Staying close to stock gear ratio will result in greater fuel mileage.

 

[rudypoochris]

my 4.0 5 speed explorer with 4.88 gears and 36s is getting 17.5 every tank.

I run 2800 at 65

Cool...?

My 4.0L 5 speed explorer with 3.27 gears and stock tires is getting 20.9mpg at 74.5 mph average speed (burst to 83, slows to 55). It pulls 23.6mpg at 66 mph (similar burst to 80, slow to 55). I don't know what it does any slower. Combined I am usually looking at ~18-19mpg with city driving. I do a lot of city. My eventual goal is to hit 30mpg at 65mph. Yes it is bold, but its what I want to do. This probably won't take me there, but I am working on a few things and just really want to run 2.73's. I will let you all know when my transmission blows up and I am getting 15mpg and what not.

So for now, I just want to put 2.73's in the car, really. I do.

Any insight at all to my issue would be really appreciated. I am sure a lot of people have a lot of opinions about what gears I should run, but I would really just like to know if anyone knows of a source of 2.73ish ratio D35 R&P's.

 

[Burns]

The questions is are you going to have the power to actually make it pull 65 or 70

 

[rudypoochris]

The questions is are you going to have the power to actually make it pull 65 or 70

Yes. I will... 65mph consumes 22.7hp purely in stock aerodynamic drag. Then lets say you bump it to approximately 40hp for rolling resistance and such, which wouldn't be that much, but we could imagine. You would still only need 128ftlbs of torque. The motor should be doing more than that at 1630rpm, if peak is 220fltbs or so at 2400rpm. I know I will have to down shift, and going to the mountains will most likely see a lot more use of 4th and below, but that is no issue. Cruising around freeway at 1800 or so is a lot nicer than the 2150 I am doing now. It isn't the end of the world.

 

[trevell407]

We arent disagreeing just for fun,
How much research did you do before deciding you want 2.73 gears?

 

[Burns]

Ok im going to put it this way.

If the people who make them have not figured this out by now. Why do you think that it will work as a modification.

I would love for you to actually prove me wrong but i do not see that happening either...

 

[4x4junkie]

My eventual goal is to hit 30mpg at 65mph. Yes it is bold, but its what I want to do.

You got some sleek teardrop shape body you plan to put on there? Because you will never reach such mileage with the aerodynamic brick that it is now.

And like everyone else is saying, these engines (and transmissions) were NOT designed to cruise at 1600 RPM.

But do as you wish.

And no, there are no 2.73 gears available for the D35.

 

[rudypoochris]

Thanks guys. I will talk to the gear shop and ask what they recommend. I might have to get unconventional on this one if I want to keep 4x4, unfortunately.

I think a T-5 swap is probably a better move compared to swapping gears, unfortunately that is expensive and requires adapters to be made which seems like a huge pain.

 

[DeRocha]

When I put on 33" tires I effectively changed my rpm to closely match the numbers you posted. In my case I had increased rolling mass and wind drag.

OEM Tires (FireStone 235/75R15 Diameter ~ 28.9")
Axle Gears: 3.27" Trans: .75:1 T-case: 1:1 @65mph = ~ 1880 rpm

Tires (BFG AT KOs 33x12.5x15 Diameter ~ 32.7")
Axle Gears: 3.27" Trans: .75:1 T-case: 1:1 @65mph = ~ 1638 rpm

I clocked 6 months of driving with 3.27s and 33" tires. From my experience city driving was fine and highway driving was barely ok provided I stayed to relatively flat areas. Acceleration was slower, but the biggest problem was trying to maintain 65mph when going up even the slightest hill. The 4.0 just doesn't have the power to do so without having to drop the transmission down a gear and give it alot of gas (just about flooring it). I stopped using cruise control as it would get overwhelmed and drop out as soon as a hill was encountered. I was very aware of the strain I was putting on my transmission and tried to baby it along, but driving in hilly areas was just brutal. IMO any fuel savings would be offset by the large amount of gas needed to make it up hills.

Re-gearing is expensive unless you have specialized tools and skill to install them. For 4x4's your front and rear gears need to be within 2% of each other to prevent driveline damage due to torque wind up issues.

Its your vehicle and obviously you can do whatever you want to it. We are hear to answer your questions and give you insite to what has been done before and what works and what doesn't. I'd like to hear you say you went ahead and regeared the rear and it is working as you have planned, but I doubt the results will be as you expect. You will be placing the engine outside of its powerband causing it to work harder. The best comparison I can give is that It will feel like a manual transmission that is always 1 or 2 gears to high.

 

[HahnsB2]

I'm also going to have to agree with everyone else, 2.73s will not help your milage, I'd bet it worsens it.

 

[Willard]

There's a reason it is already geared the way it is. Your current gears will keep the motor in a more efficient power band. Aldive, who get's 30+ mpg in his explorer, runs 3.73 gears, and has done extensive research to prove they are the best gears for gas mileage in our trucks.

It's not going to be cheap to regear both axles, and you will regret it.

It should also be noted that our transmissions will NOT be happy with extra load, and you will yours in short time. You will lose efficiency over time with the motor, as it does the motor good to turn higher RPM's for a little while sometimes. If the motor never gets to turn fast, it will build up excess amounts of carbon, and not flow well.

 

[rudypoochris]

De Rocha, just out of curiosity were you running the 5-Speed or the auto?

Could someone please tell me more about the 30+mpg Explorer running 3.73's? I am quite interested in this. Like extremely interested...

There is a 3.08 ratio if the 2.73 is excessive, but I really don't see the car having an issue keeping freeway speed with stock tire diameter and small hill work. As I said earlier, the car was geared for 55mph speed limits and it is turning below 1700 at that speed. Yes, this IS out of the power band... for a reason, power costs fuel simply put. Same reason an LS2+T56 gets 28mpg but makes 400hp. Large hills though, I agree... but since OD is .79 and 4th is 1:1 it really isn't that big of a deal to drop it one or two gears... right? Also as far as carbon build up and such goes... I rip of the car quite frequently in all honesty, it isn't an issue. I know it sounds weird what I am trying to achieve and the way I drive, but I am not an 'eco-modder'. I just realize it would be nice to pull more economy out of this truck. In the future if I want more power 2.73's should be awesome for building boost. I think the real thing the truck needs is a T-5 as those ratios are MUCH better imho, but unfortunately it needs adapters and such.

Regearing is a pain though, I do agree. As far as excessive load and all that stuff. I don't think I will be loading the transmission out of specification in all honesty. I cruise at 1000rpm @ 35mph already and I enjoy driving this way as it has saved me a ton of fuel. It would be a costly mistake to re gear and be upset about it though. I will think about it some more and crunch some numbers.