Okay guys, I know there is a ton of questions about re-gearing and which gears to choose. I did a few quick calculations based on a 4R70W or 4R75W.

My truck (06 FX4) came with 3.73 gears and 275/65/18 tires (32" tall).



Here is a link to the calculator I used.
http://www.angelfire.com/fl/procrastination/rear.html
Transmission 3rd gear is 1:1, O/D is .7:1
This is under the understanding that the 4R75W has the same gearing as the 4R70W. If I'm wrong about that, someone please correct me.

Hopefully others will use this as a guide when deciding what gearing is right for them. I used this before re-gearing my last truck and it worked out perfectly. I also checked the numbers with 3.73's and 35's at 70mph and they worked out as the calculator says.
Enjoy!
-Brian


(Here are a couple other calculators I use for tires/gearing)
http://www.therangerstation.com/tech...tioChange.html
http://www.therangerstation.com/tech...nch_Tires.html
http://www.therangerstation.com/tech...awl_Ratio.html

 

I have a formula for determining how to regear so that you are back close to stock.

(New Tire Size / OEM Size) X OEM Gears = New Gears

I started w/ 235/70/17 tires & 3.55. I went to 315/70/17 w/ a lift kit.

(34.4 / 30) X 3.55 = 4.0706666666666666666666666666667

I now have 4.10 gears.

5150

 

 

Yep, sounds about right. That's the simple way to calculate how to return to stock ratios. 3.55's with 30's are pretty close to 3.73's with 32's, and 4.10's with 35's
I generally prefer lower gearing (higher numerically) than stock when using over-sized tires to help with the extra rolling mass and drag. Especially if you're pulling hard through mud or something and need torque to keep turning the wider tires. When I re-gear it will either be 4.30's or 4.56 gears for 35's.
So anyway I put the chart up here, not just to tell people which gear ratio brings them close to stock, but to get a real-world idea of how different gearsets effect the rpms on the highway. It helped me decide last time I re-geared something.

 

When these trucks leave the factory, they are suppose to get good enough mileage for the EPA, w/out sacrificing too much power. With the little 30" tires, they could get even better mileage w/ 3.23 gears, but the truck would feel sluggish under load. For a daily commuter who doesn't ever pull anything heavy, that would be great. For those who usually load up the bed and tow, 4.10 would be a lot better but won't make the EPA happy. And believe it or not, there are still some people who look at those EPA mileage numbers on the new car sticker when they go on the lot.

 

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