3.50 ring and pinion vs 3.55 ring and pinion

JWC

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Hi everyone! I'm new two this forum but I've had my 81 Bronco since 1985. Does it cause a problem to run a 3.55 ring and pinion in the front with a 3.50 ring and pinion in the rear 9"?
 

miesk5

96 Bronco 5.0
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Yo JWC,
WELCOME!
Ford typically builds 4WD trucks with a slightly numerically lower front gear ratio than the rear so that off-road steering is enhanced. So a truck built with 3.55 rear gears will have 3.54 front;
3.08 rear - 3.07 front;
4.11 rear - 4.10 front, 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 in 4 wheel drive. 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.
Also 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.


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1981 Bronco Dealer Brochure by Gary @ 1981 Bronco Brochure - Gary's Garagemahal (the Bullnose bible)

1980-1986 Bronco/F Series Fasteners by Gary

1980-1986 Bronco/F Series Carburetors, Chokes & EFI by Gary

1980-1986 Bronco/F Series Specifications by Gary @ Axle Ratios - Gary's Garagemahal (the Bullnose bible)

Differences in 1980-1986 Bronco/F Series by Gary

1981 Bronco/F Series Electrical & Vacuum Troubleshooting Manual (EVTM), partial via Gary @ 1981 EVTM - Gary's Garagemahal (the Bullnose bible)

EEC III Testing, etc @ Wayback Machine by Tommy

Partial Chilton Manual for 1978, 1979, 1980, 1981 Bronco. Scanned by broncocity.com @ Bronco City

1981 Bronco Advertisement @ Ads0002.jpg

GL J!
Al
 

BroncoAZ

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The reality is that the ratios are the same for a 3.50 vs 3.54, but I think they number them differently to avoid mix up on part numbers. My rig has gears that were sold as 4.09 ratio D44 front and a 4.11 ratio 9” rear. There are only so many teeth on the ring gear and pinion gear, not an infinite number. If your D44 3.50 ratio front ring gear has 39 teeth and the pinion has 11 teeth and the 9” 3.55 ratio rear ring gear is the same 39 teeth and the pinion is the same 11 teeth then the ratios are identical. You would have to have a different number of teeth front and rear for the ratios to really be different.
 

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