I saw a real simple explanation of how to calculate your gearing on Xtreme 4x4 today. Both how to figure out what your big tires did to your gearing and how to figure out what gearing will take you back to stock.

I'll use my rig as an example. I went from 31" to 33" tires - rig came with 4.88 gears.


Effective Gearing - (what has going to big tires done to the gearing)

[Old size] divided by [New size] multiplied by [current gear ratio] = [New effective ratio]

Example: [31] divided by [33] x [4.88] = [4.58]


Equivalent Gearing - (what gears do I need to swap in to get back to "stock" gearing)

[New size] divided by [Old size] multiplied by [current gear ratio] = [New equivalent ratio]

Example: [33] divided by [31] x [4.88] = [5.19]

 

I also saw that show yesterday.
Thats the only piece of good info that I have taken away from watching it...

 

Yeah, no kidding....

 

So how many inches is a 235x75x15 ?

 

Here's the formula for converting metric tires to inches:

Using your numbers: 235/75R15 (235mm = width, 75 = aspect ratio - percentage of width to sidewall height, 15 = rim diameter in inches)

[metric width] divided by 25.4 gives you width in inches. (25.4mm in an inch)

Take the width in inches and multiply it by aspect ratio (ie 75 = .75, 70 = .7, etc.) which gives you (1) sidewall height.

To get diameter, take sidewall height multiply by 2 and add the rim diameter.


Example:

[235] divided by 25.4 = [9.252"] wide

[9.252"] multiplied by .75 = [6.939"] sidewall

[6.939" x 2] = [13.878"] total sidewalls

[13.878"] sidewalls + [15"] rim diameter = [28.878"] overall diameter.

Keep in mind this is how you calculate the diameter. The actual size may deviate slightly from the given dimensions.

 

So, Gregg - if you had 4.10's stock with 28.878 tires and moved to 33's, here's the effective and equivalent gearing:

Effective: 28.878/33 x 4.10 = 3.59

Equivalent: 33/28.878 x 4.10 = 4.68

 

Yes, that's my current setup. I had put in 4.88's, they were set up wrong and only lasted a month in the rear. So I bought a third setup with 5.29's from NWOR(never again) and they lasted 5 years before they broke. So I'm back to stock now. I preferred the 5.29's over the 4.88's . When I get some money again I'm going to regear again. Thanks for the math.

Your truck came stock with 4.88's?

 

Yep, 3.0 V6, auto tranny with factory alloys and 31" tires came with 4.88's stock. You can probably find some factory 3rds that would bolt in. Generally 90-95 4Runners had this setup.