Ford 9" question

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May 2, 2011 | 10:16 AM

chad.deraps

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Ford 9" question

I am going to replace the stock 2.75 gears in my 78 F150 with 3.89 gears and a detroit locker.

My first question is how do I know the bearing size without dismantling the axle?

Second, I am wondering about the gear ratio. This truck is mainly going to be used for hauling stuff. It will have a rebuilt 400 with a C6. I want to eventually put 33" tires. I calculated that at 75 mph it would be about 2900 rpm with 3.89 vs 3100 with 4.11. Any suggestions on which to use?

Thanks,
Chad

May 2, 2011 | 11:02 AM

Bullitt390

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I get 2970 at 75 mph with 33" tires... Figure some convertor slippage and you'll likely see 3100-3200 rpm at 75 mph.

Josh

May 2, 2011 | 11:48 AM

chad.deraps

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What gears are you using

May 2, 2011 | 01:05 PM

Bullitt390

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Using your numbers in my own rpm calculator.

I have used 3.00, 3.50, 3.70 and 4.11. 3.50 is by far the best all-around gear ratio IMHO

Josh

May 2, 2011 | 01:57 PM

chad.deraps

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OK, thanks for the information. What are the reasons going for the 3.5 over the 3.89? Is it just better gas mileage? Should I worry about driving at 75 mph running over 3k?

May 2, 2011 | 03:03 PM

Bullitt390

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A steady 3000 is ok, but I wouldn't go much over that.

3.50 gears with 33 inchers and torque convertor would likely hold a steady 2700-2800 at 75 which is better IMHO

Josh