Rear gearing question

My 72 was originally was a 302 car but has since had a 429 installed and I need some help on what the gearing should be any help would be great thanks

How do you want to use the car? In town smiles per mile or long distance cruising? Is the 429 stock or upgraded? There are a lot of variables here. Chuck

Also, which transmission is in the car.

Also, if you plan on doing any "high performance" driving, you may want to upgrade to a 9" rear end. It's stronger and easier to find limited slip or locker center sections for it.

rvrtrash said:

Also, if you plan on doing any "high performance" driving, you may want to upgrade to a 9" rear end. It's stronger and easier to find limited slip or locker center sections for it.

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Or an 8.8 out of an explorer. 3.73 gears, limited slip, and disc brakes in one shot.

Ok lots of questions so first, it's a daily driver so both and the engine is stock. The transmission is a c6, and the rear end is already a nine inch.

Hileanor said:

Ok lots of questions so first, it's a daily driver so both and the engine is stock. The transmission is a c6, and the rear end is already a nine inch.

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Either a 3.50:1 or 3.70:1 would be fun and fine for a daily driver. I run a 3.89:1 and think the higher 3.70:1 is a better choice for all around use.

You can see what your engine RPMs will be with different gears with this: https://spicerparts.com/calculators/engine-rpm-calculator
C6 final drive ration is 1:1.

Sheriff41 said:

Either a 3.50:1 or 3.70:1 would be fun and fine for a daily driver. I run a 3.89:1 and think the higher 3.70:1 is a better choice for all around use.

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Ok sounds good and thank you, but I do have a question I was told that they came from the factory with a 3.25 by going to a 3.70 will I have to look out for a lot or a little wheel spin under acceleration

Hileanor said:

Ok sounds good and thank you, but I do have a question I was told that they came from the factory with a 3.25 by going to a 3.70 will I have to look out for a lot or a little wheel spin under acceleration

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Just to chime in here and try not to display my ignorance, but I do know the 3.25:1 gear with the 429 was mandatory for a car with AC. If no AC, do what you want I guess. It's only money for gas at the end of the day.

Hileanor said:

Ok sounds good and thank you, but I do have a question I was told that they came from the factory with a 3.25 by going to a 3.70 will I have to look out for a lot or a little wheel spin under acceleration

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Well, that would depend on how hard you get after it! Typically you can spin the tires more with a lower gear. Personally, I like the responsiveness and acceleration with the lower gear and I only spin the tires when I want to. The seat-of-the-pants feel with a lower gear is noticeable. The factory 3.25 gear is a nice cruiser and with a 429 should effortlessly smoke the tires but the acceleration will be less impressive.

Awesome thanks for the info sounds like I will be going with the 3.70, y'all are the best

Hileanor said:

Ok sounds good and thank you, but I do have a question I was told that they came from the factory with a 3.25 by going to a 3.70 will I have to look out for a lot or a little wheel spin under acceleration

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Many Fords were only offered with certain rear end ratios because of being equipped with Air Conditioning. My Mach 1 came with A/C and factory 3.00:1 open rear end gears. I bought a whole center section complete with Traction-Loc and 3.50:1 gears and replaced the rear end I had, but, as I said ,it couldn't come factory like that as I also have A/C. The Factory wants to limit the stock compressor speed by doing so, however, many people switch to R134 retro-fit systems which use a modern Sanden compressor, and those aren't sensitive to RPM like the stock York or Tecumsah compressors .

I have a 429, close ratio 4 spd and 3.5 gear. It’s plenty of fun in town but on the freeway I am wanting an overdrive. Someday I may add a gearvendor’s overdrive.

Just going to throw another opinion in here. Since that 429 is stock, then a 3.70 gear will not be ideal. Those engines were built to roll around loafing along in a car that often weighed 4500lbs, had 2.75 gears and 28" tall tires. The 429 in my 71 Squire loves to roll at 75mph, but it's only turning 2400rpm. The typical Mustang will be running 26" tires, and at 75mph with 3.70s, that poor 429 will be spinning it's guts out at 3700rpm.

IMO, a 3.25 gear would be ideal - which would give you brisk acceleration, and reasonable rpm on the highway of around 3200.

I agree with Hemikiller. I had a '69 vintage 4v 429 with 10.5:1cr in my Mach1 with 3.50:1 rear end & wide ratio toploader. It had a mild cam & small amount of porting. Suppose it still made around the stock 480 lbs.ft torque, overall multiplication in first was 3.5 x 2.78 = 9.73, 9.73 x 480 = 4670 lbs.ft I was trying to put to the pavement. Unsurprisingly it went sideways if you tried to pull out of a junction in a hurry & would easily demolish the tyres. Average miles per gallon was 13 (UK gallons) = 10.4 with US gallons. Even on Hoosier 275x60x15 Quick Times at the strip I had to baby it off the line & shift to second as quick as I could. It was a lot of fun to drive but I'm glad I never had it out in bad weather.

Another vote for less than 3.50 gears. I have cars with 2.75, 2.80, and 3.55. The 3.55, there is no "chill" mode. You can't really just cruise around. From the moment you leave the driveway till you get back home, its Ric Flair mode.


If you dont want to always be Ric Flairing everywhere you go, look at the lower number rear ends. With a 2.75 rear, and a 8.5" wide tire, the tires will pin if you stomp on the gas. But you can still cruise at a relaxed RPM if you want.

I agree with Hemikiller. Particularly If you have the wide ratio 4 speed, The 3.25 is the best compromise.

giantpune said:

Another vote for less than 3.50 gears. I have cars with 2.75, 2.80, and 3.55. The 3.55, there is no "chill" mode. You can't really just cruise around. From the moment you leave the driveway till you get back home, its Ric Flair mode.
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If you dont want to always be Ric Flairing everywhere you go, look at the lower number rear ends. With a 2.75 rear, and a 8.5" wide tire, the tires will pin if you stomp on the gas. But you can still cruise at a relaxed RPM if you want.

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LMFAO ok you guys the Rick flair clinched it I will look for lower than a 3.50. besides I'm a macho man fan. OH YEAH!!

I really like the 3.5:1 TractionLok in our 73 Mach 1 (P235R60 x 15 tires on 15 x 7 Magnum 500 wheels). A nice level of performance without the engine sounding like it is about to grenade at 60 MPH (2,800 RPM or so). But, when the C4 began to slip I opted to swap in an AOD. Best of both worlds, a decent ratio for lower end performance, and an OD to keep the RPM<s nicely tamped down at highway speeds. The engine (351W, not C) was built for street/strip, so it has the torque available to provide a higher top end despite the added wind resistance at higher speeds.

3.25:1 is not that far away from 3.5:1, so I would also be inclined to give it a nod. A nice compromise ratio with those to gear ratios for a rear axle.