Help with rear end gear calculation?

[CaliLifeStyle]

Speed x Gear Ratio x 336 / tire diameter = RPM

I put in 80 mph x 4.10 x 336 / 25.5in = 4322 RPM's. That would most likely be 3rd gear right? I want to make sure that this is right because it could help a lot of people.

 

[CaliLifeStyle]

I got this calculation from a guy who wants to put gears in his Dodge Ram. He is going with 4.56's. I'm not sure if this calculation works for only Dodge Transmission's.

 

[tjm73]

Don't know about that one, but I know this one works...........


MPH x Gear Ratio / Tire Height x 336 = RPM

70 x 4.10 / 25.7 x 336 = 3752 RPM in 4th. 3752 x 0.67 (5th O/D ratio) = 2513 RPM

 

[CaliLifeStyle]

It came out the same so I must be doing it right. Thanks.

 

[trinity_gt]

For a TR-3650 Mustang with P245/45R17s (814 revs/mile) the following should be about right:

1st: RPM = 45.856*Speed(MPH)*AxleRatio
2nd: RPM = 27.134*Speed(MPH)*AxleRatio
3rd: RPM = 17.772*Speed(MPH)*AxleRatio
4th: RPM = 13.567*Speed(MPH)*AxleRatio
5th: RPM = 9.089*Speed(MPH)*AxleRatio

In general:

RPM = ((TireRevsPerMile*MPH) / 60) *AxleRatio * TransRatio

TireRevsPerMile can typically be found on the tire specifications sheets. It's a better term to use than the diameter because it represents the real-world, loaded and pressurized effective rolling diameter of the tire.