yo BN,
Ring and Pinion Calculator; Calculate Ratio By Counting Teeth
Ring and Pinion Calculator
or
Gear Ratio Determination; "...remove the differential cover or the 3rd member and count the number of teeth on the gears. Unfortunately, this can get messy. One somewhat easy yet very reliable way is to jack up the wheels, disconnect the driveshaft, rotate one wheel, and count how many revolutions the pinion yoke/****** makes. If you turn one tire & the other one either stays still while the pinion rotates, or the other one rotates backward while the pinion stays still, or something in between, then you've got an open diff or a very weak LSD (like a Trac-Lok with too many miles on it). In this case, you need to secure one wheel (perhaps by lowering the tire onto the ground), then you can figure your gear ratio by rotating the airborne tire TWO full rotations and counting the number of times the pinion rotates. 3.73 turns means 3.73 gears. If you only rotate the tire once, then 1.865 turns means 3.73 gears. If you turn one tire & the other one turns the same number of turns in the same direction, then you've got a good LSD or locker. In this case, you can just rotate the tires ONE full rotation and figure your gear ratio by counting the number of times your pinion rotates. 3.73 turns means 3.73 gears..." Read More
Source: by Obi-Wan at
http://www.jedi.com/obiwan/jeep/misc/gearratio.html"...
Ford built our Broncos & other 4x4 trucks & vans with a numerically lower front gear ratio in the front Dana 44 than the rear so that off-road steering is enhanced. A Bronco built with 3.55 rear ratio would have a 3.54 ration in the front Dana 44; or; 3.08 in the 8.8 & 3.07 in the Dana 44; or 4.11 in the 8.8 & 4.10 in the Dana 44, etc..."; Following was in my MS WORD Notes and the source, Randy's Ring & Pinion has removed it from their current web site; The gear ratio in the front of a four wheel drive has to be different from the front so the front wheels will pull more. There have been many different ratio combinations used in four-wheel drive vehicles, but not so that the front will pull more. Gear manufactures use different ratios for many different reasons. Some of those reasons are: strength, gear life, noise (or lack of it), geometric constraints, or simply because of the tooling they have available. I have seen Ford use a 3.50 ratio in the rear with a 3.54 in the front, or a 4.11 in the rear with a 4.09 in the front. As long as the front and rear ratios are within 1%, the vehicle works just fine on the road, and can even be as different as 2% for off-road use with no side effects. point difference in ratio is equal to 1%. To find the percentage difference in ratios it is necessary to divide, not subtract. In order to find the difference, divide one ratio by the other and look at the numbers to the right of the decimal point to see how far they vary from 1.00. For example: 3.54 ÷ 3.50 = 1.01, or 1%, not 4% different. And likewise 4.11 ÷ 4.09 = 1.005, or only a 1/2% difference. These differences are about the same as a 1/3" variation in front to rear tire height, which probably happens more often than we realize. A difference in the ratio will damage the transfer case. Any extreme difference in front and rear ratios or front and rear tire height will put undue force on the drive train. However, any difference will put strain on all parts of the drivetrain. The forces generated from the difference have to travel through the axle assemblies and the driveshafts to get to the transfer case. These excessive forces can just as easily break a front u-joint or rear spider gear as well as parts in the transfer case.
Source: by miesk5 at Ford Bronco Zone Forums
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The 9 inch will be an BIG issue because it does Not have the Vehicle Speed Sensor (VSS); also called RABS Sensor & Differential Speed Sensor (DSS)
The VSS (mounted on top of the diff and protrudinmg inside in conjunction with its Tone Ring (also called Exciter Ring), located within the diff is used to send signals (108 pulses per diff rev) through Connectors & 4WABS Control Module, etc to PSOM, PCM & E4OD; & to Cruise (Speed Control), if equipped.
But to solve this;
Speed Sensor Recalibrator for Swapping any Differential into 90-96 Bronco; "...we have been working on this for a while and it is the nicest design I have worked with. You dont have to send your ****** out for a core and the sensor spacing is adjustable and it is mathmaticaly the same as stock. $300 shipped to your door in the lower 48. It comes with 100% of the parts you need and should very easy for most people to install. With original location for ford trucks, the sensor is essentially past or after the gears but before the tires if you think of the drivetrain in a straight line, so only tire size changes affect your speedo reading.When you move the tone ring (the little ring with the magnets) up to the output shaft, you put it before the gears, so now gears and tires will affect it. Factory setup: engine-transmission/tcase------gears(diff)tone ring----axles---tires; new setup: engine-transmission/tcase-tone ring------gears(diff)----axles---tires.
And if you you give me the tire size and gear ratio i will do the math and give you the the instructions to program the speedo.. Super easy to do.
.." Source: by mkiefer (Mark Kiefer)