The 3.73 vs 3.91 Dilemma

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jerkyboy

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After years, I am now serious about changing out my 3.23 rear for a 3.73 or 3.91. I have a gen III 5.7 hemi (360 HP @ 5400 rpm/360 ft-lbs of torque @ 4200 rpm) and five speed with 3.27, 1.98, 1.34, 1, and .68 gear ratios. My rear tires are 26.7 inches tall. I drive the car on the street and highway and don't want to be buzzing at too high rpms when I get on the highway. With my 3.23 rear, I'm about 1,800/2,100 rpms at 65/75 mph.

I got on the engine rpm calculator website, which says at 65/75 mph I'd be at 2,075/2,394 rpms with 3.73s and 2,175/2,509 rpms with 3.91s. My car before this was a '63 Nova with 3.73s and a turbo 350. I remember hating to drive on the highway with it because the engine winding would drive me nuts, obviously because of no OD.

That said, can anyone with a similar setup weigh in on which may be better...also, is there a rule of thumb for rpms vs comfortable highway driving? Always appreciated:D
 
Dude the difference is like 100 rpm and you're still only at 2,500 at 75. You won't even notice the difference between the 3.73's and the 3.91's on the freeway.

I mean seriously, go for a drive, see if there's a significant difference in engine sound between 2,400 and 2,500 rpm. Kinda doubt it. But ultimately you have to decide if you can live with it.

My 340 with 26" tires and 3.55's turns just over 3,400 rpm at 75mph and it can do that all day long. I don't know that I'd want to listen to it on a 6 hour drive, but the 30 minutes I commute on the freeway is fine. What's comfortable for you and what's comfortable for the engine are different things.
 
What 72blunblu said x2

I'd go with the 3.91's It cant be that annoying at 2500rpm. And how much driving are you actually going to do at 75mph?
 
373 or 391,72 is right,you wont be able to tell the diff.god i wish i had a 5 speed in the duster.
 
If You don't plan to drag race, often, go with the 3.73.
 
Agree, little difference between the two. However, if you are not drag racing, from my experience I would go 3:73 especially with the 3:27 1st gear. Just curious why not stay with the 3:23? Also, you should be able to find a 3:91 third member (assuming you have an 8-3/4"). 3:73 is a non-factory gear set.
 
I do travel the highway often but only long stints (2.5 hours) once/year. I get that the rpm diff is minimal but was just looking for feedback from someone who has a similar setup. I will not be drag racing and want to go with a lower gear because the 3.23 doesn't have that put you in the seat feeling like my past 3.73 and 4.10 setups I've had in the past. I'm starting to think that maybe my clutch is worn as it's not easy to get the wheels to spin and no chirps between gears. Also, my 3.23 setup is a sure grip.
 
I understand your concern about 3.91s in the highway. I had them years ago and drove from my house in Pittsburgh to the Mopar Nats in Columbus, Ohio. Between the engine RPMs and the fact that I removed all of the insulation (didn't help the weight) I was NEVER so glad to get out of a car. I was sweating and my ears were ringing LOL. Theses were however NOT as bad as the 4.30s I had until I blew them up. The car now has 3.55s and although I miss the acceleration of the 4.30s its much happier on the highway and for that matter around town.
 
Current case is a 489, is a 741 compatible?

You can swap in a whole 741 3rd member into the axle, if that's what you're asking. But you can't swap gears between those cases, ie, put 741 gears in a 489 case. The 741 case is weaker than a 489, so it would be a downgrade. Probably not an issue with your horsepower levels on the street through.
 
I'd just be swapping out the 3rd member so it should be no problem. Are the u-joints the same?
 
I'd just be swapping out the 3rd member so it should be no problem. Are the u-joints the same?

There are two yoke sizes, joints are 7260 or 7290. So, the u-joints are only the same if the yoke sizes are.
 
Steve, no way to suggest anything without knowing: Primary use of car, engine/horsepower, what car, tire size, etc...

3.55 are perfect for my Dart with 26" tires and 340/727. But I'm not racing it, or taking any long highway trips.
 
Gears 3.73 or 3.91 automatic transmission

There's typically a little more drivetrain loss with an automatic, but it totally depends on your torque converter. The final transmission drive gear is 1:1 in both, so, drivetrain losses and the spread between gears are the only difference. But the torque converter will change how your engine handles both of those things. With a tight convertor the difference would be minimal.
 
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