Debbie 1 Report post Posted August 11, 2019 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 Quote Share this post Link to post Share on other sites
Brian Conway 264 Report post Posted August 11, 2019 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 Quote Share this post Link to post Share on other sites
Mach1 Driver 560 Report post Posted August 11, 2019 The gearing depends on the transmission- with an overdrive you probably want something around 3.50. Positraction is a Chevy coined word. For Ford limited slip differentials many people prefer a Detroit Trutrac by Eaton. Quote Share this post Link to post Share on other sites
Mike65 475 Report post Posted August 11, 2019 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. Quote Share this post Link to post Share on other sites
GypsyR 32 Report post Posted August 11, 2019 Kind of odd choices there. "Posi" is Chevrolet rear and "Danas" tend to be found under Dodges. What exactly do you want the car to do? What engine and transmission are in it (or will be in it?) Quote Share this post Link to post Share on other sites
mustangstofear 608 Report post Posted August 11, 2019 We basically run 3:50 most of the time. A good rule is the less horsepower you have you can make it up with lower gears. Quote Share this post Link to post Share on other sites
Vicfreg 771 Report post Posted August 19, 2019 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/ Quote Share this post Link to post Share on other sites
mauleta 10 Report post Posted August 20, 2019 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 Quote Share this post Link to post Share on other sites
Rsanter 152 Report post Posted August 21, 2019 Too little information. what do you want to use the car for? what engine? how much HP and torque? what trans? what gears are in there now? Quote Share this post Link to post Share on other sites
