What gears should I put in? I was told to put 5.13 from a buddy but local shop says to put 4.88's in it. It is a wimpy 5.4 with a 7.25" lift on it. Any help is greatly appreciated.
Thank You

 

What year truck..? 5.13 sounds good to me. Thats as high as you can go..I think!!

 

You didn't specify what year your truck is so I'll use the stock tires on my '03 in this example. They are 265/75R16. This works out to a 31.65" using the following formula:
((2 * 265 * .75) / 25.4) + 16 = 31.65"

Where 265 is the tire width and .75 is the sidewall height as a percentage of the width and 16 is the rim size. The 25.4 converts from mm to inches.

Assuming 37" is really the diameter of your new tires then the ratio of the diameters is 37 / 31.65 = 1.169.

Multiply the stock rear end ratio (3.73 in my case) by the 1.169 above to get the new rear end gears that will keep your overall ratio the same.

3.73 * 1.169 = 4.36.

If your stock rear end was 4.10 then 4.10 * 1.169 = 4.783.

Now you have the math and can plug in your actual values to find the closest set of gearing that matches what you need.

Daryl

 

I would think 4.88's should be enough with 37's. 5.13's would be good if you intend to tow alot of weight with it or like to crawl real slow over stuff offroad. If you do alot of freeway driving, the 4.88's will allow better mileage if you like to drive fast. 4.56's are another choice, but with the small 5.4L engine and heavy truck, you may find 4.56's to not be enough gear reduction if you tow or live in an area with alot of hills. I run 4.56's with my 33's for more pulling power since I haul a camper and tow a trailer and only have a 351 in the truck, but most people with 33's do fine with 4.10's. 4.56's usually work good with 35's.

 

I have 8 inches of lift - soon to be nine but I have 37's with the 5.4. I put 4.88's in it and that brought me back basically to a 4.10 rear end with stock tires ( the equivelent)

5.13's IMO would be too much. At 70 mph I am at about 2500 or so rpm. Out of OD when I am pulling a heavy load - at 70 mph it is at about 3200 rpm...I can lock on the cruise and go down the highway!

This will allow you to calculate anything you need.

http://www.ring-pinion.com/calculato...latorsind.html