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Debbie

Stock rear to posi

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Hi all

I have a 9” factory rear in my mach1 and want to put in a posi rear. I was told if I install a 411 I will lose either take off speed or top end speed so I was also told a 373 Dana will take care of take off speed and top end and suggestion?

 Thank you

Deb

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Debbie let me suggest you don't throw any of your parts away.  Ford did use a 3.50 gear set and many seem to be happy with it.  Automatics and 4 speeds ' like ' different rear gears and the type of driving are factors to consider.  Brian

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When I had the rear rebuilt in my 69 Coupe I had 3.50 gears installed along with a trutrac. I had 3.50 gears in a 68 GT Coupe I had back in the 80's & liked it, so I had the same ones installed in the 69.

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Keep the housing and the axles.  All you need to do is upgrade the "center section" to a posi-traction or "track-lok" in Ford verbiage.  Plenty of reputable suppliers can get you the new center section.  I use 3:50 gears.   4:11 are typically drag-race or street race gears, not very drivable for normal street/highway cruising, unless you make a transmission change to a later overdrive type.  

Also, make sure it is a real 9"   the 8" is very similar.  Good website here for info:    https://www.diyford.com/ford-axle-history-identification-ford-differentials/

 

 

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Debbie,

As far as the gearing goes, using a 4.11 will result in a quicker take off from a dead stop, with the result of a lower top end speed before you hit redline.  All other things being equal, a 3.73 gear will have a higher top speed but will be slower off the line than the the 4.11.  Also, you will burn more fuel when cruising using a 4.11 vs. a 3.73.  Neither are really meant for cruising, but the 3.73 is a little friendlier towards it.

Look at it this way, using a 4.11 rear gear set and a 1:1 final in the transmission, the engine will rotate 4.11 times for each revolution of the rear tires while if you have  3.73 gear set, the engine will turn 3.73 times for every revolution of the rear tires.

Mike

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