yo t,
WELCOME!
EDIT! I messed-up my C&P.
First, confirm the rear diff is an 8.8
Information, History, ID, pics, etc.; "...8.8” rear’s have a ten bolt inspection plate on the back that’s about 10” from the center of the top bolt to the bottom bolt center. The 8.8” first saw service in the 79 Crown Vic. and in 1980 for the F100/150..."
Source: by Kenneth C at
http://badshoeproductions.com/rear.html
A tip by Seabronc addressed to a different person, "...Just to add a little. In reference to your question about only changing the back gear ratio. as long as you don't use 4x4 you would be able to run just fine, but if you put it in 4x4 that way you will have an unpleasant experience.
So I would suggest you don't do that or unhook your front driveshaft. If you can't afford a front set and rear set at the same time, I would suggest that you wait till you have both sets before converting...."
Gear ratio is calculated by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear. The higher the number, the lower the ratio: a 5.29 gear has a lower ratio than a 4.10 gear. With a lower gear ratio the drive shaft (and thus the engine) turns more for each revolution of the wheel, delivering more power and torque to the wheel for any given speed. Lower ratios are generally desirable when going off-road. Higher ratios are better for freeway driving since they run at lower RPM’s and offer better fuel economy.
Changing tire size affects the final drive ratio. Switching from a 30" tire to a 35" tire changes the final drive ratio by about 17%. This may drop the engine out of its' "power band" and result in poor performance and fuel economy. To restore performance you must change the gear ratio to compensate for the change in tire size. If you originally had 3.07 gears you need a ratio that is approximately 17% lower, such as 3.55. If you want to increase off road performance you might want a 4.10 or lower ratio.
by Randy's Ring & Pinion
Do I have a traction device such as a positraction or locker in my vehicle?
Put the transmission in neutral and jack up both tires. Turn one tire. If the other tire spins the opposite direction then you have an open differential, and if it spins the same direction then you do have a traction device.
http://www.ringpinion.com/TechnicalHelp/TechHelp.aspx#13
Jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posi’s and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.
http://www.differentials.com/technical-help/faqs#What gear ratio do I have?
"...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
