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Old 09-03-2012, 08:41 AM
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700 r4-rearend gearing

Hello everybody, happy labor day to ya'll. Well I would like to pick your brains on a gearing issue. I have a 1973 Laguna that about 4000 pounds. It's pushed by a 454 and has a 700r4 with a 2800 stall connected to a ford 9 inch with 4.10 gears. I recently lost the pinion bearing in the rearend so now that I have it torn down I am considering changing the gear in the rearend. When I cruise around town the o/d is causing the engine to lug between 45-55 mph so I shift into third and then it is running to high for my comfort. My question is should I go up in gear to a 4:56 so the o/d is useful in town driving or go down to 3:73, 3:50 so I can use 3rd gear and leave the o/d for expressway driving. By the way I have 28 inch tires but I may change those down the road as well. Any opinion is welcome, thank you.

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Old 09-03-2012, 09:08 AM
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I would go to the 3.50 gear if you have a mild cam. Overdrive is for highway use.
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Old 09-03-2012, 10:58 AM
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thanks gearhead, I am a little leary about going to far down in gear because I like being able to melt the tires on occasion. Another concern is if I have it into o/d it might really lug in local driving. Any thoughts?
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Old 09-03-2012, 11:08 AM
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Most automatic transmissions have a first gear ratio between 2.48 and 2.50. The 700R4 has a first gear ratio of 3.06. If you had a TH350 with a rear end ratio of 4.10 the final drive would be about 10.16. With the 700R4 and a rear end ratio of 3.50 the final drive ratio would be 10.71, so you would have a better first gear ratio with the 700R4 and 3.50 gears than someone with a TH350 and 4.10 gears. The 454 should make enough torque to melt the tires plenty well with the 700R4 and 3.50 gears. As I said above, overdrive is for highway use, not in town. But in the end this is a decision that you have to make yourself.
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Old 09-03-2012, 11:18 AM
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Appreciate the info. This is a great website for guys like me that like to dabble but don't know the intimate details of gearing. I think I'm going to check into the 3.5 gear. Thanks a bunch.
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