440/727/3.91 suregrip...who's got it and who likes it?

[challngd73]

OK.  My 741 carrier is howling, and my father offered up his new 742 carrier with 3.91 suregrip.  My car currently has a 3.23 suregrip.  Anyone using this setup?  how do you like it, and how is driveability?  I dont do any expressway driving, just tool around town with some 60-65 driving on back roads.  Plan on putting a 5 speed in next season and the motor will then be a 512.  Will 1st gear in a tko 5speed even be useable with that combo?  anyways, who's using 3.91's and whats your advice?  thanks..

[dodge freak 2]

I'v got a 3.91 gear but with just a hot little 318. Had it for over 10 years. Had 28 inch tires and now have 26 inchers.

It buzzes alright on the freeways. I say up to 45 mph its really nice. 55 mph is starting to get up there and 70 mph its buzzing. But with a 6,500 red line triple digit speeds were no problem.

Me liked going from 28 to 26, I felt the difference but lost a little top speed.

If you can run a taller tire it help. A 29 or 30 inch tire I say it be ok up to 65 mph.

I do sometimes wish I went with 3.55, then again some times I wished I went with 4,10's.

More gearing makes everything easier my trans guy used to say.

In 2nd gear you have like a 4.68 gear in back. Put your trans in 2 gear and drive it steady for 10 mins at 55 mph--that is how 65 mph be like with 3.91's in 3rd gear (auto trans)..regardless of tire size. 

[moparmike70]

I've got 3.91's with the stock F-70 15's which are like 25-26in tall. The gear is great around town, absolutely love it. I too don't do much driving on the highway. When I do though it's tough. On the highway 65mph is top speed except for quick bursts. I have a 383 with a 4-speed. You should be fine with the 5-speed first gear and will have the best of both worlds. The 3.91's will probably be a little much for your 512 unless you build it to rev. The 512 will have more than enough torque that you'll have a tough time taking off without blowin the tires off.

[Blue Stone]

 I have a 4:10 Dana  and wish I could get a 3:91 seems like the ultimate in between gear......

[Cooter]

When I ran with the "5.0" croud, I ran 4.10's with a T-5 5-speed with that crazy 3.35 First gear ratio..It was GREAT out the hole, but you needed to get that sucker in 2nd Quick....There's only like around 100 RPM's or so difference between 4.10's and 3.91's...I'd go with the 3.55's and the 5-speed..You will not need that much gear when your motor is puttin out like 650 Ft. Lbs. of torque....

[challngd73]

Yeah, I think my plan will be to use the 3.91's right now, then have the 3.23 gears put in when I get the 512 done.

[drewcrane]

i have exactly that combo,440 ,727, 3:91,

instant tire smoke,tons of low end,

if you want traction you will have to feather the throttle a little,

but dont grab second in a corner or you will spin out,

and yes  hiway cruising is well a little revved up,so the need for o/d is an added plus,

put em in and go have fun you will smoke alot of cars light to light

[challngd73]

i have exactly that combo,440 ,727, 3:91,

instant tire smoke,tons of low end,

if you want traction you will have to feather the throttle a little,

but dont grab second in a corner or you will spin out,

and yes  hiway cruising is well a little revved up,so the need for o/d is an added plus,

put em in and go have fun you will smoke alot of cars light to light

Awesome, thanks for the reply!  What rpm are you seeing at 60?

[drewcrane]

some where around 3500,to 3800 depending on tire size,

i just changed to a 3:23, for a vanishing point cruise,and kinda miss the low end pure power,
 i have had 3:55,s also and might go back to those

 but i can get more traction with the current 3:23,s and i want to do more long cruises ,

so if you are not planning any road trips go low and like i said have some fun scaring the  out of your passengers

[ntstlgl1970]

I have had that combo in my car for the last couple years before I took it apart for paint. The freeway rpm will vary a couple hundred based on what torque converter you have and the rear tire height. On mine, with a 27" tall tire and a 2800rpm converter I would be at ~34-3600rpm on the freeway at 68-70 mph. Also I agree with cooter regarding your first gear ratiowith the 3.91, it will be very short- like 1-2 shift would be 1/2 way or less through an intersection from a stop. Depending on what you want to do. I used to work on a ton of 5.0 mustangs and the best all around gear IMO for the 5 speed was the 3.55 with short tires (23-24") 3.73 with taller tires and 4.10s for strictly drag racing.

[challngd73]

My tires are 27.5 inches, and my torque converter is stock..

[ntstlgl1970]

Should work good with your combo, and with the 5 speed when you go that route.

[drewcrane]

here use this ,it is on this site it works very well

http://vexer.com/automotive-tools/speed-rpm-calculator