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I have a 87 Bronco on 35" tires that I think my be geared to low. the tag on the rear diff has two numbers the top number is S842J and the bottom is 273885C27 I need someone to decode this so I know what I have. I would also greatly appreaciate some suggestions on what my gearing should be. Has c6 trans.
 

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Driving Stuff Henry Built
-90 xlt, 351w, e4od, man 1356, 3.55, sag, warn hubs, 35s. -73, 400, np435, d20j twin, 35s
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Please excuse the cut & paste, but here's a bunch of info that I'm just too lazy to repeat. :toothless
13- What do the axle gear ratio numbers mean? (3.55:1, 4.10:1, etc.)
14- Which gears are higher or lower?
15- What gear ratio do I have?
16- If I change the rear gears do I have to change the front too?
17- How do I choose the right gears?

13- It describes how much gear reduction there is in the axle. Gear ratios are normally separated by a colon like this 3.55:1. The 3.55 refers to the number of revolutions that the driveshaft turns, while the 1 refers to revolutions of the axle (Or tires). The driveshaft turns 3.55 times for 1 revolution of the tires. 4.56:1 gears would have the driveshaft turn 4.56 times for 1 revolution of the tires. So with 4.56:1 gears, the driveshaft turns more times for each rotation of the tires than with 3.55:1 gears. That gives the engine more leverage to turn the tires easier.

If you opened up the differential & counted the number of teeth on the gears you would find the ratio by comparing the number of teeth on the pinion gear (Attached to the driveshaft) to the ring gear (Attached to the axle). You divide the number of pinion teeth into the number of ring teeth. So if you divided 32 ring teeth by 9 pinion teeth you would have a 3.55:1 ratio. 41 ring teeth divided by 10 pinion teeth would be a 4.10:1 ratio, & so on.


14- The terms higher gears & lower gears have become confusing. You think of 1st gear in your transmission as being a lower gear than 3rd. Using the same principle, 4.56:1 gears are lower than 3.55:1. But numerically they're higher. 4.56 is a higher number than 3.55, even though 4.56 gears are lower. So we have to clarify how the gears are being described. Frequently you'll see the term "Lower gears (Higher numerically)" used to clarify the meaning.


15-
The easiest way to find gear ratios is to decode the door sticker. There's a chart quoted below to look up the codes. But it's only a good solution if the door sticker is still readable & no one has swapped the gears previously. One accurate approach is to open the diff & count the teeth as described above.

An easier way is to mark the driveshaft & a tire. Use the usual safety procedures (Flat solid ground, blocked wheels, solid axle stands, etc.) & raise the rear of the truck off the ground. Turn the tires 1 revolution by hand & count the driveshaft revolutions. About 3-1/2 turns of the driveshaft would be 3.50 or 3.55:1. A little over 4 driveshaft turns would be 4.10:1 & so on.

Another easier option with an open diff is to raise only 1 rear tire. Turn that tire 2 revolutions by hand & count the driveshaft revolutions. With the spider gears in the differential, 2 revolutions of 1 tire is the same as 1 revolution of 2 tires. So with 2 revolutions of 1 tire & about 3-1/2 turns of the driveshaft would be 3.50 or 3.55:1. A little over 4 driveshaft turns would be 4.10:1.

Note: 2 digit limited slip codes refer to the rear axle only, 3 digit limited slip axle codes refer to both front & rear axles.
So H9 is LS rear only, & H92 is LS front & rear.

16- Yes, they need to match. If front & rear gears are different, front & rear tires try to travel a different distance for each revolution of the driveshaft. Something has to give. If tires don't slip, some point in the drivetrain will break.

There is a tiny percentage of difference that's acceptable. Stock gears on many later Broncos are 3.55:1 rear & 3.54:1 front, due to the different manufacturers of components. Combining 3.55:1 & 3.73:1 would be too much of a difference.


17- It'll vary according to your usage. If you drive a lot of highway you might want higher gears (Lower numerically :toothless) than if your truck is primarily a wheeler.

A good place to start is by using what the truck had stock, or what you have now, as a baseline. It was designed to run well in many different situations with stock gears & tires. So if you increase tire size & regear so that you have the same RPM at the same speed as stock, you have a good starting point. Once you have that point, you can adjust to favor your type of driving. Lower gears (Higher numerically) for more power to the tires for hill climbing or towing. Higher gears (Lower numerically) for lower RPMs on the highway. Many of us would rather run lower gears with more power to the wheels, but it should to be tailored to your specific needs.

This Gear Ratio Calculator at grimmjeeper.com allows you to compare 2 setups at once. Enter your starting point on one side & compare it to different options on the other.

With my 90 as an example, I entered my e4od trans, the BW1356 transfer case, & 3.55 stock gears on both sides. On one side I showed the stock 29" tires & on the other the 33" tires that I have now. I hit calculate & it tells me that at 65mph originally it ran at 1898rpm & now it runs at 1668rpm. I can sure feel that difference on uphills. I downshift more now than when it had 29s on it, & I use 4low long before others when wheeling. I need lower gears (Higher numerically).

If I change to 4.10:1 gears, it shows that I would run at 1927rpm at 65mph. So that would be a little lower than stock gearing with 33s on the truck. That would be nice for my current usage, but since I'm considering 35s or 37s in the future, I compared some combos with both 35s & 37s. Here's some outcomes:
3.55:1 & 29s = 1898rpm at 65mph, stock.
3.55:1 & 33s = 1688rpm at 65mph, higher gearing than stock. Current combo.
4.10:1 & 33s = 1927rpm at 65mph, slightly lower gearing but close to stock.
4.10:1 & 35s = 1816rpm at 65mph, higher gearing than stock.
4.56:1 & 35s = 2020rpm at 65mph, lower gearing than stock.
4.56:1 & 37s = 1911rpm at 65mph, slightly lower gearing but close to stock.

So for my use, 4.56 is probably where I'll go, since it looks like it would work with either 35s or 37s. If I wanted more power to the wheels for hill climbing, I might go even lower (Higher numerically).

Another thing to consider is that if you increase the tire size, you give the tires more leverage. By lowering the gears, you give the engine & driveshaft more leverage to turn those big tires. By giving both the tires & engine/driveshaft more leverage, the parts in between are more likely to break. After a certain point those components in between will need to be improved to survive.
 
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