Need some help finding out what size Ring Gear Diameter I have, 9.75" or 10.5"? Probably going to a 4:10 to get some power back after lifting my truck. Thanks in advance for the help.

Current Axle Ratio: 3:55 LS
Bolt Count: 12

 

I'm pretty sure that would have a 8.8" ring gear.

 

You have the 9.75 ring gear. 10 bolts is the 8.8" ring gear. You can also look on the differential tag, the series of numbers on your tag should read 3L55 975

 

Depending on your tire size you might want to move up to 4.30s if they have them.

 

this.

with 35's 4.30's put you closest to stock gear ratio. IIRC it puts you almost dead on, the only reason most guys go to 4.56 is they don't have a 4.30 reverse cut for the front diff.

 

He has 2wd so that would be perfect.

 

i'm %90 sure the 4.6's came with the 8.8, but like mentioned above 430's would be perfect

 

in the 04-08 thats true, not sure on 09+ but i won't be surprised if they phased it out of the f150

 

ohh ok true, I have seen 09-10's with the 8.8

If the diff looks like this its an 8.8


This is a 9.75

 

B.S. He has 3.55's now and he went from a 32" tire to a 35", if you do the math he only needs to go to a 3.88 gear to get back what he lost from the 35" tire. Since they don't make a 3.88 gear, you go to the next higher ratio, which would be a 4.10, and the extra gear will compensate for the increase in weight of the larger tire. A 4.10 gear is more than sufficient.


35 x 3.55 = 124.25 / 32 = 3.88

Just saw he is 2wd so he had maybe a 31" tire stock so the formula would put him at 4.00 for the gear he needs for the 35's

 

yes, but 3.55's are crap to begin with for power which that 4.6 is going to need help. Have you had a 4.6 with 35's? probably not. I am recommending what i think is best after having a similar set up. and alot of guys come from 3.73's so 4.30s are what will be perfect

 

Upon further review of Randy's Ring and Pinion website they don't offer a 4:30 for the 08-10 9.75" Rear End. They go from a 4:11 to a 4:56....

 

me personally i'd go 4:56

 

According to Tire Rack, the 2009 STX had 2 different tire choices from factory. Either a 255/65/17 (30.1") or 265/60/18 (30.6") .... For argumaent sake, I'm going to assume that he had the 17 inch wheel option from factory.

I am useless otherwise, but assuming the above formula is the correct one to use, 35 x 3.55 = 124.25 / 30.1 = 4.12

So would a 4.10 gear almost but not quite make up for what he lost, and ultimately leave him a little extra short considering the extra weight of the tires?

 

OEM Tires: 265/60/18 = 30.5 Michelin LTX MS2
New Tires: 325/65/18 = 34.8 Nitto Terra Grapplers

34.8 x 3.55 = 123.54 / 30.5 = 4.05

Yukon 4:11 + SCT Tuned =

Note: I don't hardly ever haul or tow anything