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66-96 Ford Broncos - Early & Full Size
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Guest Monsterbug_willy

38" tires on stock bronco any issues?

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Guest Monsterbug_willy

Hi all, I have a 1990 bronco and got my eye on some 38" geolander tires. I got 33"s on right now with a 6" Bilstein lift but ive read some things about going past 33-35" you start snapping axle shafts and other issues that come up. What else should I be worried about breaking by putting 38" tires on there? I feel like it would stress a lot of parts especially since it's almost 30 years old... Stock axles and everything 4x4 5.8l V8

Thanks! Will

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Yo Will,

Welcome! 

With the humongous 38s, the speedometer driven gear and both differential gearing are all based on the distance the tire travels during one complete revolution. This is based on the outside diameter of the tire-and-wheel combo. If the change in the diameter of the tire is large enough, the speedometer will be inaccurate, it will indicate a speed slower than your actual speed and 0-60 tines will be equal to a Yugo?.

When you change the diameter of your tires drastically, you could change all these settings and impair the performance and safety of your Bronco. if you don’t modify other components accordingly, such as;  gear ratios, front and rear differentials should/must be changed.   See Gear Ratio Guide for Larger Tires @ http://web.archive.org/web/20060115054646/http://www.roughcountry.com/Gear Ratio Guide.htm

Speedometer Gear Change in a 90 (in Xfer Case Tail Housing) Source: by Keith L (TTB Blows, Bling-Bling) @ https://www.supermotors.net/clubs/superford/registry/139/13464-2

Somehow, the font changed to red!

"...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. by me

Some gear swaps for example;

4:88 Installation Info & pics in a 96 Dana 44 
Source: by Jer C (Godless, Beerman, SOLD) at SuperMotors.net 
5.13 Installation pics in a 96 Dana 44 & 8.8 
Source: by Al S (White Cloud, OwlStu) at SuperMotors.net

GL and have fun!

Al 

 

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