72' rear gear change

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   Hello everyone!
I'm going to change my differential. I've read and researched a lot, but I'm not quite sure in a few things and trust in your experiences. Currently, a 9" Ford axle with a gear ratio of 2.75:1 is installed without a lock. Engine is a modified 351C and a revised 3-speed Fmx automatic transmission with 2400 stall speed converter. The camshaft has a range from 1800 to 5600 rpm. And my tires have 26.1" in diameter. I would like to switch to a 3.25:1 or 3.40:1 with lock. I tend to tend to a 3.40:1, since the speed increases only slightly and the price remains the same. However, I would not like to scrap or overstrain my newly built engine. It happens from time to time that I drive on the highway with a maximum of 85-90 mph. But it is rare. Mostly around 70 mph on the secondary road. I have already tried some computers on the Internet regarding the change of speed. The following is the result with 90mph to have air upwards:
2.75:1= 3186 rpm (currently installed)
3.25:1= 3765 rpm
3.55:1= 3939 rpm
Are these calculated correctly? Are these speeds unproblematic for my engine? Maybe someone has a similar setup?
Is it necessary to adjust the shiftpoints on the transmission?
Furthermore,
 I would like to know what drive gear is installed in a standard FMX, so I
 change the driven gear on the speedocable that the right speed is 
indicated. Are 8 teeth right? Or do i see it when the speedocable is off the transmission?

I am looking forward to your experiences or recommendations and feedback! Thanks a lot!
Thomas

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tom72torino Members Profile Send Private Message Find Members Posts Add to Buddy List New Member Joined: 18-March-2016 Location: Austria Status: Offline Points: 24	Post Options Post Reply Quote tom72torino Report Post Thanks(0) Quote Reply Topic: 72' rear gear change Posted: 24-January-2017 at 7:27PM
	Hello everyone! I'm going to change my differential. I've read and researched a lot, but I'm not quite sure in a few things and trust in your experiences. Currently, a 9" Ford axle with a gear ratio of 2.75:1 is installed without a lock. Engine is a modified 351C and a revised 3-speed Fmx automatic transmission with 2400 stall speed converter. The camshaft has a range from 1800 to 5600 rpm. And my tires have 26.1" in diameter. I would like to switch to a 3.25:1 or 3.40:1 with lock. I tend to tend to a 3.40:1, since the speed increases only slightly and the price remains the same. However, I would not like to scrap or overstrain my newly built engine. It happens from time to time that I drive on the highway with a maximum of 85-90 mph. But it is rare. Mostly around 70 mph on the secondary road. I have already tried some computers on the Internet regarding the change of speed. The following is the result with 90mph to have air upwards: 2.75:1= 3186 rpm (currently installed) 3.25:1= 3765 rpm 3.55:1= 3939 rpm Are these calculated correctly? Are these speeds unproblematic for my engine? Maybe someone has a similar setup? Is it necessary to adjust the shiftpoints on the transmission? Furthermore, I would like to know what drive gear is installed in a standard FMX, so I change the driven gear on the speedocable that the right speed is indicated. Are 8 teeth right? Or do i see it when the speedocable is off the transmission? I am looking forward to your experiences or recommendations and feedback! Thanks a lot! Thomas
	Have a nice day! Greetings from austria! Thomas

72FordGTS Members Profile Send Private Message Find Members Posts Add to Buddy List Admin Group GTS.org Admin Joined: 06-September-2005 Location: Ontario, Canada Status: Offline Points: 5846	Post Options Post Reply Quote 72FordGTS Report Post Thanks(0) Quote Reply Posted: 26-January-2017 at 3:08PM
	You should be fine with any of your ratios you listed. Ford offered a 3.50:1 gear ratio with the 351-4V engine. How long do you plan to drive at a sustained 90 MPH? Your RPM calculations seem fine, except for the 3.55:1. That should be 3.40:1 for 3939 RPM. Keep in mind the actual RPM may be slightly higher due to some torque converter slippage.
	Vince 1972 Ford GTS Sportsroof - Survivor, One Family car GTS.org Admin

californiajohnny Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 05-October-2013 Location: winlock, wa Status: Offline Points: 14609	Post Options Post Reply Quote californiajohnny Report Post Thanks(0) Quote Reply Posted: 26-January-2017 at 4:05PM
	tom, yes the FMX came with a 8 tooth drive gear (it's machined into the shaft) mine came with 2.75 and it had a 17 tooth driven gear on the cable. i think if you go with a 3.25 may be good, lower gear for take off without sacrificing the top end too much.
	JOHN 74 GRAN TORINO S&H CLONE 74 VETTE CUSTOM 90 S10 BLAZER 4X4 LIFTED 77 CELICA CUSTOM 75 V8 MONZA SUPERCHARGED 79 COURIER VERT. SLAMMED 75 VEGA V6 5 SPD 70 CHEV C10 P/U 68 MUSTANG FB CONVERSION

lynchster Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 07-January-2006 Location: Pennsylvania Status: Offline Points: 2150	Post Options Post Reply Quote lynchster Report Post Thanks(0) Quote Reply Posted: 26-January-2017 at 6:40PM
	Back when I was driving mine I was happy with the 3.50 gears. The 4.11 gears were a bit much for highway speeds.
	*Chuck* 72 Gran Torino Sport 13 Taurus SHO "Mr Pig"

tom72torino Members Profile Send Private Message Find Members Posts Add to Buddy List New Member Joined: 18-March-2016 Location: Austria Status: Offline Points: 24	Post Options Post Reply Quote tom72torino Report Post Thanks(0) Quote Reply Posted: 27-January-2017 at 7:23PM
	Thanks a lot guys! I've seen that I'm mistaken. The 3939 rpm are for the 3:40. Thanks Vince! The 90 mph I have only calculated as security upward. Will never occur in the normal case. On our highways in Austria are 81.25 mph maximum speed. If I drive with this speed I have approximately 3556 rpm ... plus still a few rpm because of the converter. As I rarely ride on the highway, I think my decision is to fall to 3.40:1 ... Thanks for the info regarding the drive gear teeth. Then I can also order the right pinion for the speedocable.
	Have a nice day! Greetings from austria! Thomas

Billy C Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 10-February-2010 Location: Pittsburgh, PA Status: Offline Points: 948	Post Options Post Reply Quote Billy C Report Post Thanks(0) Quote Reply Posted: 28-January-2017 at 4:35PM
	The 3.25 would be a comfortable gear. The limited slip dif is a game changer. Totally awesome! I suggest a helical if you are going aftermarket.
	-Billy Conturo

torino335 Members Profile Send Private Message Find Members Posts Add to Buddy List Member Joined: 27-December-2016 Location: ca Status: Offline Points: 119	Post Options Post Reply Quote torino335 Report Post Thanks(0) Quote Reply Posted: 28-January-2017 at 6:02PM
	as billy c said,a 3.25 is a nice ratio,my last 72 had 3,25 ratio,i thought it was great all around,