High RPM on highway 1999 4-cyl - JeepForum.com
 
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post #1 of 7 Old 08-23-2014, 02:28 PM Thread Starter
gjohns25
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High RPM on highway 1999 4-cyl

I recently bought a 1999 automatic Jeep Wrangler 4-cyl and while driving on the highway the RPM read around 4000-4500 going about 65-70mph. This seems high to me, but this is my first jeep. Is this normal? If not, what could be causing this?

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post #2 of 7 Old 08-23-2014, 02:57 PM
Wheelin98TJ
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RPM should be closer to 3,400 at 70 MPH with stock size tires and stock axle gear ratios.

What size tires do you have?

Are you sure the speedo is accurate?
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post #3 of 7 Old 08-23-2014, 03:02 PM Thread Starter
gjohns25
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Quote:
Originally Posted by Wheelin98TJ View Post
RPM should be closer to 3,400 at 70 MPH with stock size tires and stock axle gear ratios.

What size tires do you have?

Are you sure the speedo is accurate?
Tires are 31x10.50-15
Not sure if those are stock sized
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post #4 of 7 Old 08-23-2014, 05:19 PM
Wheelin98TJ
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31 x 10 50 is not the stock size. It is taller and wider.

The taller tires would drop your rpm at highway speeds.

You may have a different gear ratio in the axles.

Rotate your tires one complete revolution while counting driveshaft revolutions. That'll tell you the axle gear ratio. Or if you are due for a diff fluid change, open up the diff and find the ring and pinion tooth count stamped on the side of the ring gear. There is more info in the FAQ sticky thread on how to determine your gear ratio if I'm not being clear.
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post #5 of 7 Old 09-01-2014, 04:38 PM
chuck85225
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With the same tires you have I'm at 3000-3100 rpm at 70 with stock 3.73 in my 98
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post #6 of 7 Old 09-01-2014, 04:46 PM
afusco1222
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Has the Jeep ever been lifted or have bigger tires on it? Sounds like the PO had upgrades on it, and took them off to sell the Jeep.
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post #7 of 7 Old 09-01-2014, 06:53 PM
2jhanna
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Quote:
Originally Posted by afusco1222 View Post
Has the Jeep ever been lifted or have bigger tires on it? Sounds like the PO had upgrades on it, and took them off to sell the Jeep.
This was my thinking also. Maybe had bigger tires and lower gears. Then took the tires off to sale and left the lower gears in.

jhanna
denton county, tx
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