3.91 good or bad

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6

69gtdart

Guest
hey every one i just picked up a 8.75 3.91 sure grip for my dart it has a 318 and a 4 speed i am curious how they ae going to perform on the freeway does any one else have this combo and how do you like them
 
they will suck on the freeway. i had them in my dart for a few years. hated them things. great around town but even with a 28" tire it was either the slow lane or cruise at 3800 rpm and man will is suck gas...
 
abodyjoe said:
they will suck on the freeway. i had them in my dart for a few years. hated them things. great around town but even with a 28" tire it was either the slow lane or cruise at 3800 rpm and man will is suck gas...
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Agreed on all above. I run a 27" rear tire with my 3.91.


But it sure makes "Smiles-per-Mile" around town.. :toothy10:

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3.91 is about as steep as I'll go on a street car that has to use the highway. I had them for years too, and I didn't mind them but the rpms can get up there keeping up with traffic.
 
My 3:21's with a 28" tire are at about 2600 at 110km's(65mph)however I can drop into second at this speed and only go to about 3400-3500 rpm,great fun.
 
I run 3.91's behind the 440 auto in the 73 Duster. My driveway is not quite 100 yds long and at the end i will be in third gear. Not a high speed rear but like said earlier. It'll make ya smile BIG.
 
if it is an every day driver you will get tried of them .... if it's a weekender some of the best times you can have with your pants on ....3000 at 60 just at the rpm that i starte to make hp at
 
I love mine, I have a set in my 69 Roadrunner daily driver. 12 miles each way to work on the freeway. I even have short tires, 195/75r14s. I think the motor doesn't care if you are turning 3000 or 2600. If it does then I would worry about 1200 rpm.
 
I've got 4.11s and they suck on the highway. In fact, they kind of suck in 45mph speed zones.

Great for the track or around town, but much too much for a pure street car.
 
If it's just a short time on the freeway, you'll be able to live with them but long distance.....no way.....but the onramps are fun! :)
 
3.55s would be better, even a 3.73 is pushing it unless you have 28" + tires on a street car for cruising. I had 4.30 and like Joe said you are looking at darn near 4000 rpms at the highway. Good for a reving 1/4 or 1/8 car, or a 3/4 ton truck with big tires etc...

Look for 3.55s
 
thanks every one how much could i sell the third member for it is a 742 case or is there a good place to buy just gears
 
it will cost you more than 400$ to buy gears bearings and set up the rear end. I like to keep a variety of rear ends for different moods. you can sell and buy for even money if that's what you want to do. They aren't that hard to swap out for a variety.You can buy one complete for the cost of changing that one over. then you'd have two. I have a rebuilt 742 3.23 clutch stile id trade even but then you wouldn't have 3.91s any more.
 
Have an original 3.91:1 geared sure-grip A-body 8-3/4" rear (now has LBP axles) in our supercharged 1972 Dart. The car has 245/60R-14 tires/wheels.

I hate them.

At 55 MPH the car feels/sounds like it's in second gear!
Really sucks the fuel down too.
Though as others have stated, step on the go pedal and it's almost worth it, almost.

I'd like to have something a little more tame.
Probably end up go to a 245/60R-15 tire/wheel combo and a 3.55:1 (maybe, but unlikely 3.73:1) gear ratio.
 
I'm thinking of running 3.91's in my 70 Swinger. Hooked up behind a TKO 5 speed, should make a very comfortable drive on the highway in 5th gear. Had a TKO in my 69 Camaro and it was a blast to drive, and actually increased the gas mileage by almost double on the highway--in town in lower gears was a different story though! Ben.
 
bmaudlin said:
I'm thinking of running 3.91's in my 70 Swinger. Hooked up behind a TKO 5 speed, should make a very comfortable drive on the highway in 5th gear. Had a TKO in my 69 Camaro and it was a blast to drive, and actually increased the gas mileage by almost double on the highway--in town in lower gears was a different story though! Ben.
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O/D transmissions throw an entirely new factor into the equation.
With the .68 final drive in that TKO 5-speed (rear tire size & cruising speed dependant) 4.10:1 and even 4.56:1 would work well on the highway.



1:1 final drive + 3.91:1 gears + 27.1" tires (255/60R-15) = ~2,900 RPM @ 60 MPH
.68 final drive + 3.91:1 gears + 27.1" tires (255/60R-15) = ~2,000 RPM @ 60 MPH
.68 final drive + 4.30:1 gears + 27.1" tires (255/60R-15) = ~2,175 RPM @ 60 MPH


.68 final drive + 4.56:1 gears + 27.1" tires (255/60R-15) = ~2,300 RPM @ 60 MPH
1:1 final drive + 3.23:1 gears + 27.1" tires (255/60R-15) = ~2,400 RPM @ 60 MPH
1:1 final drive + 3.55:1 gears + 27.1" tires (255/60R-15) = ~2,640 RPM @ 60 MPH
 
There are calculators online all you have to do is plug in your ratio, tire size and final trans ratio.
 
I like mine!:-D Most of our highways are 55-65mph anyway.

I had 4.10's in my old Duster340 which was a bit much for the highway I felt even though I drove 250 miles one time at 55-60mph.:-D
 
Years ago my ex-wife was driving the stock AAR with 391`s to nursing school one week a month at 100 miles a day. The week that she drove the car it used a quart of oil. I could not figure out why; because I drove it hard and street raced it, and it did not use any oil with me driving it. Come to find out ,she was driving the whole distance at 90 to 95 at 5-55 hundered on the tac, and the other girls were scared to death when it was her week to drive. Another thing when the car shifted to second you could not prevent it from chirping the tires at any RPM. Well I changed all of that by pulling the rear and setting up the sure grip with 276`s. At 60 MPH it is now 2100 and i/4 mile I don`t need 3rd gear. I don`t think that it lost anything except a fender off the line and gained an unreal top end.When the car gets restored, I will put the 391`s back in. Now days I don`t drive like that and thankfully God allowed me to live through those times. Wayne
 
I had a 3.91 in my 1966 340 Barracuda with a 4spd for several years. In town it was OK, but I drove it to The Nationals (130 miles every August) and I was winding waaaay too tight keeping up with 70 mph expressway traffic. I'm surprised I had no overheating problem. The noise made conversations impossible. We won't even talk about gas mileage! The 3.91 is in the shed now. I'm driving a much more pleasant 3.23.
 
daniel_depetro said:
O/D transmissions throw an entirely new factor into the equation.
With the .68 final drive in that TKO 5-speed (rear tire size & cruising speed dependant) 4.10:1 and even 4.56:1 would work well on the highway.



1:1 final drive + 3.91:1 gears + 27.1" tires (255/60R-15) = ~2,900 RPM @ 60 MPH
.68 final drive + 3.91:1 gears + 27.1" tires (255/60R-15) = ~2,000 RPM @ 60 MPH
.68 final drive + 4.30:1 gears + 27.1" tires (255/60R-15) = ~2,175 RPM @ 60 MPH


.68 final drive + 4.56:1 gears + 27.1" tires (255/60R-15) = ~2,300 RPM @ 60 MPH
1:1 final drive + 3.23:1 gears + 27.1" tires (255/60R-15) = ~2,400 RPM @ 60 MPH
1:1 final drive + 3.55:1 gears + 27.1" tires (255/60R-15) = ~2,640 RPM @ 60 MPH
Click to expand...

Thanks for the info. What I dont get is that every one here hates on 3.91s and praises 3.55 yet there is only a 240 RPM difference. Just goes to show.
 
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