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Old 10-20-2013, 09:21 AM   #1
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Question Axle Ratio vs. Top Gear Ratio

I have been reading various comments made in several threads, both here and at other sites, and I have a question.

Does the factory change the top gear ratio of the transmissions, to compensate for the axle ratio, such that you always end up with the same engine RPM at, say, 60 mph regardless if you have a 3.21, 3.73 or 4.10 rear end? If true, I would assume to attempt to maintain the same highway mpg rating.

Does anyone know?

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Old 10-20-2013, 11:14 AM   #2
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Nope. 5th gear auto in the 2012+ JKs is 0.83:1. 4th (the highest gear on pre-2012 models) is 0.69:1. That doesn't change depending on your diff gearing.

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Old 10-20-2013, 12:58 PM   #3
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Yup, Marinehawk is right,the lower the rear end gearing, the higher the engine will rev at a given rpm. The effect on fuel economy may be only modest though. It would be interesting if the fuel economy was rated for the different axle ratios.
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Old 10-20-2013, 02:41 PM   #4
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Thanks for the replies.

I had a hunch it was that way. I guess those others either were mistaken at what exact rpm their engines ran at 60 mph, or mistaken at what their axle ratio was.
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Old 10-20-2013, 03:46 PM   #5
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In case it's relevant, tire diameter will affect the RPMs at a given mph.
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