rear axle ratio change better towing capacity

2stroked wrote:

SoCalDesertRider wrote:
I would go with 4.88's. I went from 3.55 to 4.56 in my F350 with a 351 and I wish I had gone with 4.88's instead. Only reason I stopped at 4.56 is my truck has a C6 auto, which doesn't have an overdrive gear. If it had the ZF 5speed or E40D 4speed, both of which have overdrive, I would have gone with 4.88's in a new york minute!

I'm also going to change to my other F350 with 7.3 diesel and E4OD from 4.10 to 4.56, at some time in the future.

Might be overkill going 4:88, wish there was a way to try the different ratios, I will go to a bigger tire if I keep the truck 285 from a 265/75 17 which will make the gearing a bit taller

It would be impractical to try a bunch of different gear sets in your truck, but you can play with the following formulas to see how different ratios look on paper, as to tire diameter, axle ratio, engine rpm, road speed, torque and horsepower. I did a lot of number crunching before I decided on a ratio, from available ratios for my particular front/rear axle model combination.

It helps to know your engines horsepower and torque output information, particularly the graphical curve info that gives you it's output at a range of rpm's from idle to redline. Also helps to know your tire diameter in inches, or revs per mile, for your particular tires, as well as your transmission's gear ratios, 1st through overdrive.

HP = (Torque x RPM) / 5252
Torque = (HP x 5252) / RPM

In the following formulas with '336', substitute '355', for any auto tranny without a locking torque converter (ie: Ford C4/C6, GM TH350/400, Chry A727)

To solve for road speed when you know engine rpms, axle & trans gearing and tire size, use:

MPH = (RPM x TireDiameter) / (AxleRatio x TransRatio x 336)
MPH = (RPM x 60) / (AxleRatio x TransRatio x RevsPerMile)


To solve for engine rpms when you know road speed, axle & trans ratios and tire size, use:

RPM = (MPH x AxleRatio x TransRatio x 336) / TireDiameter)
RPM = (MPH x AxleRatio x TransRatio x RevsPerMile) / 60
RPM = (MPH x AxleRatio x TransRatio) / (TireDiameter x 3.1416 x 63360 x 60)


To solve for overall gear ratio (axle and tranny ratios together), when you know road speed, engine rpms and tire size, use:

(AxleRatio x TransRatio) = (RPM x TireDiameter) / (MPH x 336)
(AxleRatio x TransRatio) = (RPM x 60) / (MPH x RevsPerMile)


To derive tire diameter in inches from manufacturer stated revs per mile, or to drive revs per mile from tire diameter, use:

TireDiameter = 20168 / RevsPerMile
RevsPerMile = 20168 / TireDiameter
RevsPerMile = 63360 / (TireDiameter x 3.1416)


To find which axle ratio would be needed to correct for a change in tire diameter, use:

NewAxleRatio = (NewTireDiameter / OldTireDiameter) x OldAxleRatio


To find tire diameter in inches, from a metric tire size, use:

TireDiameter = ((2 x SectionWidth x AspectRatio) / 2540)) + WheelDiameter


Nomenclature of a metric tire size, for use with formula, above:

Example tire size: 265/75-16E

-265 is the Section Width, in millimeters
-75 is the Aspect Ratio, expressed as a percentage of the section width
-16 is the Wheel Diameter, in inches
-E is the Load Range of the tire (tire construction and load weight carrying capacity class)