What rear end ratio do I have?

[jsimpson]

At 70mph, what rpm should I be turning with a 4. 10? With a 3. 54?

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[KatDiesel]

Originally posted by jsimpson

At 70mph, what rpm should I be turning with a 4. 10? With a 3. 54?

Tire size would be helpful to know in this equation.

 

[klenger]

You will probably find the answer here.



http://dodgeram.org/tech/specs/axle/rpm_vs_mph.htm

 

[Steve M]

RPM=MPH X gear ratio X O/D ratio X 336 /tire diameter.

On my truck, at 70mph, I would calculate...

70mph X 3. 54 X . 69 X 336 / 30. 5 = 1883rpm.

 

['956Wheel]

There is a sticker on the underside of the hood near the latch it will have the info.



Mike

 

[Cooker]

The plate held on by the diff cover bolts also tell the axle ratio. If I remember correclty it's the first three numbers.



-Ryan

 

[Blkdawdg]

Lookie Here

There is also a sticker on the door of the glovebox that will tell you what ratio you have. It would be easier than doing all of those calculations.

 

[RustyJC]

Our 2002 3500 6-speed with 4. 10's - 6th gear:



60 MPH = 2000 RPM



70 MPH = 2350 RPM



Rusty

 

[jsimpson]

245-85/16

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[MidwestMotorman]

old trick

IF:



1. You have stock tires

2. Are in a 1:1 drive ratio (fourth in a manual, and third with overdrive DISengaged)



THEN:



You will be 3540 rpm with 3. 54 and 4100 rpm with 4. 10



Just another old trick that can be used on any automobile if you can verify conditions one and two, while going 75mph. You may be off a little bit due to tire pressure and wear, but the formula holds true.