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Discussion Starter #1
hey guys I need to know what is the normal rpm of our truck, it seems that mine is always at 2000 to 2300 to 2500 rpm even if I'm cruising at 80 mph. Is that normal? I don't have any performance mod except for the 2* advance. if its not normal what should I do?
Thanks!
 

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At 80 you should be at or above 2k...
 

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At 80 mph. my truck is running 2300 rpm`s......
 

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Here is what I get from calculating:

For Big Tow in 5th gear with 285/70/17 tires, Vehicle speed in MPH = .034756 x RPM

This means that Vehicle Speed/.034756 = rpm

80/.034756 = 2302 RPM

75/.034756 = 2158 RPM

70/.034756 = 2014 RPM

60/.034756 = 1726 RPM

The calculation is:

Speed = (Tire circumference x Engine speed) / (Transmission ratio x rear axle ratio)

Tire circumference = 102.758 inches / (12 x 5280) = 0.0016218 miles

You have to multiple the numerator by 60 to convert RPM to Revs per hour.

The denominator for 5th gear and Big Tow is (.834 x 3.357)


Now you would need to correct the foregoing if your tire size is different.

For Big Tow in 5th gear with 285/70/17 tires, Vehicle speed in MPH = .034756 x RPM

To convert this to 305/45/22 gives a circumference of 103.066 (from an Internet calculator) as compared to 102.758 for the OEM tire size, which means you are going a little further for each rev so your speed at any given RPM goes up a little bit. So to convert you need to multiply by the ratio of 103.066/102.758 x .034756 = .034860

So .034860 x RPM should equal your speed in MPH in 5th gear. 80/.034860 = 2295 RPM.
 

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Wow that was crazy math..
 

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ProjectTitan007 said:
Wow that was crazy math..
Yea, I had to take 3 Tylenol after reading that one.
 

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Discussion Starter #8
bestatchess said:
Here is what I get from calculating:

For Big Tow in 5th gear with 285/70/17 tires, Vehicle speed in MPH = .034756 x RPM

This means that Vehicle Speed/.034756 = rpm

80/.034756 = 2302 RPM

75/.034756 = 2158 RPM

70/.034756 = 2014 RPM

60/.034756 = 1726 RPM

The calculation is:

Speed = (Tire circumference x Engine speed) / (Transmission ratio x rear axle ratio)

Tire circumference = 102.758 inches / (12 x 5280) = 0.0016218 miles

You have to multiple the numerator by 60 to convert RPM to Revs per hour.

The denominator for 5th gear and Big Tow is (.834 x 3.357)


Now you would need to correct the foregoing if your tire size is different.

For Big Tow in 5th gear with 285/70/17 tires, Vehicle speed in MPH = .034756 x RPM

To convert this to 305/45/22 gives a circumference of 103.066 (from an Internet calculator) as compared to 102.758 for the OEM tire size, which means you are going a little further for each rev so your speed at any given RPM goes up a little bit. So to convert you need to multiply by the ratio of 103.066/102.758 x .034756 = .034860

So .034860 x RPM should equal your speed in MPH in 5th gear. 80/.034860 = 2295 RPM.
now thats what i call a response! i thought i will never get a response. anyways thanks you so much for all your response, i thought i'm the only one having this rpm rate. its normal then. thanks again.

thread closed!!!
 
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