Anyone ever count their gear Ratio?

[Bigtman07]

Has anyone ever pulled their diff cover and actually calculated the true gear ratio they have? I am still puzzled on mine. When I changed the front diff fluid on my truck out of curiosity I wanted to see if my build sheet matched my actual truck. I have a tag on the rear axle that says Jasper Transmissions. So I think that the rear Axle has been replaced. But everything works. My sheet says I have 3.92 gears. BUT when I calculated the gear ratio I got something like a 3.69

Ok I know a 3.69 doesn't exist in our truck and factory didn't have anything of the sort. Any thoughts on to why the calculations came out odd? The 4x4 works great so I know the gears match. But it's just puzzling.

 

[Joshwaa]

Sure you did you maths correctly? I have not heard of a 3.69 either.

 

[Bigtman07]

I checked it twice. Had a buddy count and calculate and got the same number. You take the ring gear and divide by the pinion right?

 

[Joshwaa]

Yea that's how you do it. Very odd.

Now you gonna make me want to go count mine. I 3.92's out in the garage in boxes I will check them later.

 

[Bigtman07]

That's what I thought. I'm trying to remember the exact numbers.

 

[Joshwaa]

I have my stock set of 3.92 out in the garage I will go count them in a bit.

 

[Bigtman07]

Awesome! I'll see if those numbers ring a bell.

 

[Bigtman07]

Bump for weekenders!

 

[Graygoose]

If its driveable, drive at 60 MPH and see what RPM's is, then figure tire height, use online calculator.

 

[Bigtman07]

ok.... Is it actual tire height of the converted. Cause my tires aren't what they are converted to be.

You have a link?

 

[Bigtman07]

Ok on my way home I was at 60mph at roughly 1750 to 1775.

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[Graygoose]

what size tires?

 

[Graygoose]

Calculators

 

[Bigtman07]

Tires are 31" 285/75/16

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[iSlacker816]

Pretty sure 285/75/16 are 33's

 

[BlackedOutHemi]

Yep. They're 33s. 75% of 285 is about 214mm which is about 8.5 in.


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[Bigtman07]

Nope 31.5" I'll take a pic of the sidewall and the tape measure when I get the truck tomorrow.

I thought the sane thing. But after taking the tape measure to them made me rethink the size.
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[BlackedOutHemi]

Well. The math shows they're 33" lol. Maybe the tread is worn or something but that's what they're supposed to be.


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[Bigtman07]

I know. And the tread cant be 2 inches deep.

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[Graygoose]

by the calc I posted, you have 33" tall tires