Diff gears

[aisawalsh]

So i'm thinking about rebuilding the front and rear diffs but not sure what gears I should install. My bronco has a stroked 302 with a newly rebuilt AOD transmission; it will be mostly a pavement princess/daily driver. The guy who is rebuilding the transmission says 3:73 would work well with the transmission gearing.

Thoughts???

 

[needabronco]

Couple of questions. What size tires are you going to run, how fast are you wanting to cruise on the highway, and what gears do you have now?

 

[aisawalsh]

I run a 32" tire and usually cruise at around 70mph on the highway. I'm not positive what gears are installed presently.

 

[needabronco]

I'd say minimum 4.10's or 4.30's. But frankly 4.56's would be a better choice for all around power and drive ability. You'll want around 2500 or so RPM's on the highway, it takes torque to push a sideways barn door down the freeway.

 

[Viperwolf1]

3.73 is not going to be enough gear. That would put the engine around 1800 rpm at 70 mph. It takes a lot of torque at 1800 rpm to push a Bronco sized hole in the air at 70 mph. You should be looking in the 4.30-4.56 range if you want to make good use of that OD gear.

 

[Apogee]

I would go with 4.56's and call it a day if you think you'll never run a larger tire. If you might step up to 33's or larger, I'd go deeper with the gears now given that you won't lose much drivability either way. If you had a 351W I might go 4.30's on the low side due to the additional torque...but maybe not.

Below are numbers I pulled from an online calculator for the different ratios.

3.73 -> 1837 RPM @ 70 MPH
4.11 -> 2024 RPM @ 70 MPH
4.30 -> 2118 RPM @ 70 MPH
4.56 -> 2246 RPM @ 70 MPH
4.88 -> 2403 RPM @ 70 MPH
5.13 -> 2526 RPM @ 70 MPH

 

[aisawalsh]

Thank you guys for your advice, you have been a big help.

 

[ntsqd]

I note the OP says "stroked 302", just how big is it? Depending on size the torque could be approaching that of a 351W.

From experience with a 302 and AOD, though not in an EB, 1800 is right around the drone RPM, stay away! I found it to run better starting about 2100 RPM (mild cam, idle barely ever so slightly loped). FWIW I ran 3.80 gears with something like a 275/60R15 on the rear of that car, so 3.73 with a 32" tire is definitely tall.

 

[Broncobowsher]

Also depends on how the stroker was built. Most of the strokers are built with higher RPM powerband.

But I will still agree with others. 4.56 gears are probably going to be a good pick.

I was running 4.88 gears with slightly less overdrive (.70 instead of the AODs .667) and with 33" tires I still considered it a good highway cruising gear with a 351 built for torque. 3.73 gears are going to be a complete joke in a Bronco. 3.73 might be a good highway gear if you didn't have an overdrive.

 

[Apogee]

Thank you guys for your advice, you have been a big help.

FWIW, that chart assumes a 1:1 final drive ratio, so it does not include your .67:1 OD ratio of the AOD. If you want that chart to apply with the OD taken into consideration, than you need to multiply those RPM values by 0.67 for it to be correct. It is useful to find out what RPM you'll be at when the transmission downshifts, as that can impact the driveability somewhat for passing, hills, etc depending on what part of the country you drive in.

 

[sykanr0ng]

Calculators to compare gears with different tires and/or different transmissions:

http://www.grimmjeeper.com/gears.html

http://www.crawlpedia.com/calculators.htm

It may take a little Google-fu to find the ratios of some transmissions, but the more commonly used on a Bronco are listed here:

http://classicbroncos.com/tech/early-bronco-transmission-guide-manual-options

http://classicbroncos.com/tech/early-bronco-transmission-guide-automatic-options

 

[Boss Hugg]

I supposed I'll chime in and say that I just recently got my explorer swap with the .67 OD running. I'm at around 1800 at 75 with 33s. In fact, I can run it up to 80 in direct (3rd) and then let it slide into OD but then it takes an easy right foot to keep it at 75 without making it downshift. I'm looking forward to building a new d44 so I can slide some 4.56s in the rear. But I'm considering going to 35s so I might go to 4.88s.


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[Broncobowsher]

Remember that 4.56 gears in overdrive will act like a set of 3.04 gears.
3.04 would make a good freeway gear so long as the tire isn't too tall (32s should be fine).

You can't compare the gearing to that of a Mustang or even later model trucks. The Bronco has so much more drag (rolling and aero) that what you can get away with in a modern vehicle doesn't apply correctly to a Bronco. That's why you keep hearing us preach "don't fear the gear". Because we have done this before, and we have found that a lot more gear then normally found in other late model stuff is what really does work well in a Bronco.

 

[SHX669]

I'll add my 2 1/2 cents { inflation } if you stay with 32"s the 4.10s would probably be ok especially if you don't do a lot of Freeway/OD driving. I'm running 35"s with 4.10s but that's a little tall for trail use - no issue on the highway but I've got a 351w and no overdrive .
I try to avoid these "what gear " questions . Too many variables - tire size , engine build , driving habits , driving environment, definition of Daily driver . And as far as the " Don't fear the gear " I've had rigs with too low gear ratio - hated em on the highway took half the fun out of driving it . Stop light to stop light to pizza parlor to gas station ISN'T highway driving.

 

[englewoodcowboy]

1800 is the drone zone for sure. As Viper and others have pointed, these motors, no matter what displacement you have them at love the 2100-2200 cruise range at 70, it works very well for our little boxes. I have a 347 and an AOD and am running 4.88 gears and 35" tires. My cruise at 70 mph is 2197RPM and it handles my little wind pusher just fine. Somehow I am managing 15 mpg at that as well...

 

[blubuckaroo]

I'll add my 2 1/2 cents { inflation } if you stay with 32"s the 4.10s would probably be ok especially if you don't do a lot of Freeway/OD driving. I'm running 35"s with 4.10s but that's a little tall for trail use - no issue on the highway but I've got a 351w and no overdrive .
I try to avoid these "what gear " questions . Too many variables - tire size , engine build , driving habits , driving environment, definition of Daily driver . And as far as the " Don't fear the gear " I've had rigs with too low gear ratio - hated em on the highway took half the fun out of driving it . Stop light to stop light to pizza parlor to gas station ISN'T highway driving.

I've got to agree with that!
I regeared to 4.11s with 32" tires. The wife drove it and asked why it didn't shift into high gear anymore.
Sure, it made for great throttle response, but it was useless on the highway.
Goes to show...try before you buy.
It's funny how some owners will defend their investments and give that advice.%)

 

[Broncobowsher]

Please remember the first post. There is an overdrive transmission involved.
To get the 4.11 non-overdrive gearing equilivent with the AOD you would need a 6.17 gear.

 

[blubuckaroo]

Please remember the first post. There is an overdrive transmission involved.
To get the 4.11 non-overdrive gearing equilivent with the AOD you would need a 6.17 gear.

Oops!
Point taken.
The overdrive ratios of Ford's autos is pretty high too.

 

[Apogee]

Please remember the first post. There is an overdrive transmission involved.
To get the 4.11 non-overdrive gearing equilivent with the AOD you would need a 6.17 gear.

2.5 ton Rockwells come with 6.72:1...problem solved.

 

[Broncobowsher]

32" tires and rockwells, all 3" of ground clearance.