Author Topic: Cobra Jet gear ratio  (Read 1897 times)

Offline Angela

  • Hero Member
  • *****
  • Posts: 834
Cobra Jet gear ratio
« on: September 28, 2016, 08:13:56 AM »
I realize the topic of which gear ratio to use is probably one of the most beat-to-death topics in this hobby. But, after much debate I cannot make up my mind and am looking for input.
I'm restoring my '70 rear axle which was originally equipped with traction Lok, 3.50 gear ratio, 31-spline. The engine is a 428 with a 4-speed.
The goal with this car is to be able to drive it at reasonable highway speeds (60-65mph) for short distances (30-40 minute drive). There is no intent of ever drag racing the car. Accordingly, the 3.50 ratio is not an option.
I cannot decide whether or not to use a 3.25 or 3.40 gear ratio. Both options are readily available. The 3.25 will most significantly lower RPM at highway speeds. The 3.40, however, seems like a very compromise between the factory 3.50 and the less "torquey" 3.25.

Using several online calculators, I think the 3.25 is going to yield ~2500-2600 rpm, while the 3.40s will yield around 2900-3000 rpm.

I'm looking for feedback as to what's the maximum RPM I should be trying to target with a 428. I'm a bit nervous to be running close to 3000 down the highway.

Offline rockhouse66

  • Hero Member
  • *****
  • Posts: 927
Re: Cobra Jet gear ratio
« Reply #1 on: September 28, 2016, 08:58:01 AM »
I would encourage you to stay with the 3.50 gear.  Even with the nice torque of the 428, that tall 1st gear in the big spline toploader is less fun to drive with any gear taller than a 3.50 IMO.
Jim
'66 GT FB

Offline Brian Conway

  • Hero Member
  • *****
  • Posts: 1623
Re: Cobra Jet gear ratio
« Reply #2 on: September 28, 2016, 09:07:22 AM »
Running the same drive train as you; 428, 4 speed, traction-loc, 9", stock wheels with 215-70-14 tires.  Switched the 4.11's for 3.25's 5 or six years ago.  Little disappointing when I go to the track; 9.5 and 77 mph for the 1/8 mile and never get out of second.   The 3.25 gear set is just fine for in town stop light to stop light driving. First gear is no longer the ' off the line ' gear but plenty of power and response other wise.  Freeway 3200 rpm at 70 mph and 4000 rpm at 90 mph.  In San Diego in order to get anywhere the freeway is the only option and for me the 3.25 gear set is a good compromise.  I have never spoken with anyone running a 3.40 set up so no help there.  Brian
5RO9A GT  4 Spd Built 5/29/65
9TO2R SCJ 4 Spd Built 9/19/68
Owner Driver Mechanic
San Diego, Ca.

Offline Mark69Sportsroof

  • Full Member
  • ***
  • Posts: 120
Re: Cobra Jet gear ratio
« Reply #3 on: September 28, 2016, 11:13:52 PM »
Ok, I'm not sure why wouldn't the 3:50 gears work?  Thanks Mark
Mark
1969 Ford Mustang Sportsroof, 390-4V
Dearborn built on Oct 31, 1968
1973 Ford Mustang Mach 1, 351C-4V
Dearborn built on Oct 31, 1972