3.91 good or bad

-
No problem.

The calculator says it's technically about a 265 RPM difference, which might not sound like much, but can make quite a difference.

In my 1972 Dart I currently have 3.91:1 with 235/60R-14 tires.
I plan on going to 3.55:1 and also with a slightly taller tire (245/60R-15) which will help that much more.

1:1 final drive + 3.91:1 gears + 25.1" tires (235/60R-14) = ~3,140 RPM @ 60 MPH
1:1 final drive + 3.55:1 gears + 26.6" tires (245/60R-15) = ~2,690 RPM @ 60 MPH

That's a 450 RPM difference at 60 MPH!



If I went with the 3.73:1...

1:1 final drive + 3.73:1 gears + 26.6" tires (245/60R-15) = ~2,830 RPM @ 60 MPH

310 RPM difference from the current 3.91:1 set up
&
140 RPM more then if I went with the 3.55:1 combo.
 
I have ran 391's with big blocks and 15 inch tires for years and love it.And yes I do drive it on the freeway 6 1/2 hour trip to the Nats every year.
 
I don't consider a 6.5 hour trip once a year "I drive it on the freeway".
To me that's like saying I have a race car because I take my street cars down the strip once a year.
That's my opinion.

Obviously the amount and type of driving one does is what will make the difference.
If you are just driving your prized vehicle a couple weekends a month to car shows, local gatherings, and to the closest Sonic then your threshold for tolerance is generally a bit higher.
Our cars are true daily drivers. Everyday my dad or I is driving one of these cars somewhere.
Not being able to have a conversation without yelling, and running the car at 3,150 RPMs @ 60 MPH everywhere we go gets a bit old, extremely quick.
Taking a couple hundred mile trip on the weekends is not a rare occurrence in any of our cars.
 
I have 3:55's in my 340/ 4 spd/275/60/15 and 3:91 in my 440/auto/255/60/15 car. My favorite gear of all time is definitely the 3:55, the lower gears I've tried on the street were fun for a while but not something I would do again.

3:55 is probably your best bet for street cruising, light highway.
 
Hate the 391's the motor just screams at you going down the
road.Been using 355's for ever and the best set up I ever had
is in my 71 Demon 355's with a 5 speed tranny.5th gear
around 1800 rpm on the highway and it will run at 75 80 MPH
all day without the need for earplugs,If anyone ever gets a
chance to drive an old MOPAR with a 5 speed do it you'll
love it
 
daniel_depetro said:
I don't consider a 6.5 hour trip once a year "I drive it on the freeway".
To me that's like saying I have a race car because I take my street cars down the strip once a year.
That's my opinion.

Obviously the amount and type of driving one does is what will make the difference.
If you are just driving your prized vehicle a couple weekends a month to car shows, local gatherings, and to the closest Sonic then your threshold for tolerance is generally a bit higher.
Our cars are true daily drivers. Everyday my dad or I is driving one of these cars somewhere.
Not being able to have a conversation without yelling, and running the car at 3,150 RPMs @ 60 MPH everywhere we go gets a bit old, extremely quick.
Taking a couple hundred mile trip on the weekends is not a rare occurrence in any of our cars.
Click to expand...

I drive my 69 383 Roadrunner 200 miles a week mostly freeway driving. I love my 3.91s, no biggy running the motor a little faster. I'm not tired of it yet!
 
Had 3.91's in my previous duster and my present one for awhile loved them...could only go 60-65 on the highway with occasional burst to 100 mph fine with me...
 
You must log in or register to reply here.
-

Back
Top