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RPM chart doesnt match reality


 
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wts
Member


Joined: 17 Mar 2008

PostPosted: Mon Mar 17, 2008 5:52 pm    Post subject: RPM chart doesnt match reality Reply with quote

I'm new around here and Search didnt find "rpmchart" so I dont know where this is supposed to be discussed.

The http://www.jeephorizons.com/tech/rpmchart.html indicates engine speed should be 2400rpm at 55mph with 35's and 4.56's.
Is the chart using uncorrected speedo gears for some ranges?

My '94 YJ turns 1900rpm during steady-state cruising on level ground in 5th with the factory OD and a 37 tooth speedometer gear.
GPS verfies the measured MPH matches indicated speed.

1900 is about what it ran bone stock with 30" tires and 3.07 gears with a 30 tooth gear.

What gives or am I just being a tool ?Dont answer that!
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ThePhantum
Pissed-Off Admin
Pissed-Off Admin


Joined: 23 Jan 2004
Location: I knew it...I'm surrounded by Assholes!

PostPosted: Mon Mar 17, 2008 7:21 pm    Post subject: Reply with quote

Like most charts, it assumes a transmission high gear ratio of 1:1, which results in a final gear ratio equal to the axle ratio. However, most modern transmissions run a high gear that is taller than 1:1, resulting in a final gear ratio that is taller than the axle ratio. Without knowing the exact final drive ration of your (or anyones) specific application, any RPM chart is a just guideline...not a rule. The chart also clearly states that this needs to be considered:

Quote:
When selecting a gear ratio, you should consider tire size, transmission ratio, final gear ratio and engine RPM at cruise speed.


In other words, you should always do the math for yourself using the following formula to correct for the specific application:

Rpm = (mph x final gear ratio x 336) / Tire diameter
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wts
Member


Joined: 17 Mar 2008

PostPosted: Mon Mar 17, 2008 7:42 pm    Post subject: Reply with quote

Whew! Good!

Thanks.

Since most modern transmissions do have an OD gear (or two) it might be a good idea to plainly state that this chart cant include all the permutations and simply assumes a 1:1 gear which is frequently "4th" in a 5speed.

Or to multiple the RPM in the chart by the OD ratio of the gear being used at 55mph. 2400 * .81 = 1950.

Here am I'm on my 2nd post and pickin nits already!
Mr. Green
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hutchman
Captain Kirk
Captain Kirk


Joined: 07 Feb 2007
Location: Kennewick, WA

PostPosted: Mon Mar 17, 2008 8:50 pm    Post subject: Reply with quote

Put it in 4th and check it.......it should be much closer.
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