3.55 vs 3.91

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DartVadar

DartVadar

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I am looking for some new gears for my dart, and am not sure what would be best. I have a rebuilt 360, comp xe268 cam eddy heads etc, so its got some power. A factory torque converter (want to change out soon) and a 999 trans with a low first gear set, and 28 inch tall tires.

I know I need at least 3.55 rear gears in the 8 3/4 to make the combo even work at all, the 2.76 gears are doing nothing for me in the "seat of the pants" off the line. I know you can get 3.73 gears but were not factory gears I don't think. So that leaves the 3.91s. I typically only drive in the city, rarely up to 60 mph. I am kind of leaning towards the 3.91s at this point in time. For city driving without much highway (if any) and runs at the track would 3.91s be good? Or would my engine be revving really high all the time?

I also would venture to guess that there would be a substantial difference in acceleration going from 2.76 to 3.55s or 3.91s?
 
Personally I would go with the 3.91 gears .

But that's just me and I have 4.30's in my street car
 
For the track my 3:55 keep my RPMs on the low side crossing the 1/4 mile stripe,
but I also drive the car on the freeway and personally due to the fact that I do on occasion get on the freeway I wouldn't go steeper.

Having 4.30 gears in on the street is great, I bet that hustles.
 
If you rarely drive faster than 60 and performance is what you want 3.91's sounds like the way to go. Yes, either gear set would make a huge improvement over the 2.76's you have now.
 
I think 3.55's with the A999 would be a great all-around gear to use. Especially if you're mainly street driving with trips on the freeway. And you'll definitely feel the difference from 2.76 gears.

I currently have 3.23 's with A999 and 26.5 (245/60R15) tires and my RPM settles around 2700 at 65 mph. I can still squeal the tire off the line if I wanted to.

I'm planning on putting 3.55's later on and using 27 inch tires (255/60R15 tires). That should put at about 2900 at 65 mph.
Long term planning for gear vendors should put that RPM down even further at 2300. (but at $2800 I'm only dreamin' for now)
 
I do like my 3:55s, but if i had it to do over again, i would go for the 3:91s.
I have almost the same combo as you, and 99% of my driving is done in town.
I thought that i was going to do some highway driving, but i don't really......
Go for the 3:91s.
 
BB73Challenger said:
For the track my 3:55 keep my RPMs on the low side crossing the 1/4 mile stripe,
but I also drive the car on the freeway and personally due to the fact that I do on occasion get on the freeway I wouldn't go steeper.

Having 4.30 gears in on the street is great, I bet that hustles.
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Had 4:30 gears and loved them. The off the line acceleration was AWESOME! Unfortunately I broke them doing a smokey burnout for a friend son. I put in a set of 3:55s and while they are nice on the highway I want my 4:30s back. By next cruise season I will have them repaired.
 
If you have the A999 with the low gear set (first and second) then the 3.55 will feel like a 3.91 in a 'regular' automatic. I think it is about a 20% difference but I don't recall exactly. If you will be spending any significant amount of time over 50 mph, go with the 3.55.
I have 3.91 gears in my 65 Plymouth and have gone on a few 300+ mile trips. (I have had these gears since 1985) It can be a little monotonous but something you just live with.
 
3.91 gears could be seen as (nearby) city-gears.
3.23-3.55 can be seen as mild (musclecar) performance gears.
 
DartVadar said:
I am looking for some new gears for my dart,

I know you can get 3.73 gears but were not factory gears I don't think.

So that leaves the 3.91s. I typically only drive in the city, rarely up to 60 mph. I am kind of leaning towards the 3.91s at this point in time. For city driving without much highway (if any) and runs at the track would 3.91s be good? Or would my engine be revving really high all the time?I also would venture to guess that there would be a substantial difference in acceleration going from 2.76 to 3.55s or 3.91s?
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The magical question is what tires will you be running? As in what height since that will determine the actual gear ratio.

3:73's are an aftermarket gear ratio. I had it in my old 8.75.

I'd go 3:91's unless you run a very short tire.

Get your self a GVOD and run some big gears! I love running the interstate with my 4.56's ;-)
 
what's the cams power range??

with a 3.91 and 26" tall tire my dart shifting at 5900 would go through the traps at 6000 rpm..
 
2.45 x 3.55 = 8.70 torque multiplication

2.45 x 3.91 = 9.58 torque multiplication

2.74 x 3.55 = 9.73 torque multiplication

2.74 x 3.91 = 10.71 torque multiplication

Easy peasy, looks like 3.55s with low first gear is very close to launch of standard gear with 3.91s
 
My experience with those two gears has always ben that the 3.91 does not add near the RPM that it does torque. It's a good tradeoff.
 
I vote 3.91's, I just took the 355's out of my aspen. They are a decent in between gear for highway and street, but if performance is what you really want you will be happy with the 3.91's or even 4.10. not much difference between the two.
 

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here's what I think. be honest. I think sometimes, some people put too much emphasis on numbers that you drive yourself nuts trying to pick the right combo. I have done it and still doing it. Ya talk about street driving, maybe some highway and then you have to throw in the track. There is no way in hell you are going to be able to pick and be satisfied with all three. Especially with the track. If you just like to drive your car, pick a gear that is midway, affordable, drive it and love it. There are guys on here that say my combo is not a good one but I think they are wrong. I don't take it to the track. I can sit at a light and practically turn it around on take off. I can cruise on the interstate if I want. so just go with the 3.55 and love what you have. just enjoy driving the car. That's what it is all about. you can ask 10 people on here and get 10 different answers. just my .02 worth.
 
trudysduster said:
here's what I think. be honest. I think sometimes, some people put too much emphasis on numbers that you drive yourself nuts trying to pick the right combo. I have done it and still doing it. Ya talk about street driving, maybe some highway and then you have to throw in the track. There is no way in hell you are going to be able to pick and be satisfied with all three. Especially with the track. If you just like to drive your car, pick a gear that is midway, affordable, drive it and love it. There are guys on here that say my combo is not a good one but I think they are wrong. I don't take it to the track. I can sit at a light and practically turn it around on take off. I can cruise on the interstate if I want. so just go with the 3.55 and love what you have. just enjoy driving the car. That's what it is all about. you can ask 10 people on here and get 10 different answers. just my .02 worth.
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I agree 100%

On a side note if you have a performance exhaust and you do decide to drive on the freeeway with 3.91's, be prepared it is not going to be a quiet ride.
 
3.91's. Get used to the revs being up there all the time. But you did say rarely over 60.
 
FredS said:
3.91's. Get used to the revs being up there all the time. But you did say rarely over 60.
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2800 RPM is 60mph. That isn't bad. For every inch of tire change you'll see 100RPm difference. .
 
The main thing I use the car for is to drive to school and occasional driving around town, so like I said before I don't ever really drive on the highway, and the highways here are max 65 MPH anyways, so the speed limits are a bit lower anyways, I would spend most of my time at 45-50 MPH max anyways.

So I guess the 3.91s would be the best choice for what I want (acceleration over speed) If anything I can add a bit more tire on the rear, which I may do anyways because the new rear end can accommodate a bigger tire. Because currently the car isnt the funnest to drive, the 2.76s suck the life outa it with my engine combo, just dont want the 3.55s to only be a marginal increase in performance.
 
DartVadar said:
The main thing I use the car for is to drive to school and occasional driving around town, so like I said before I don't ever really drive on the highway, and the highways here are max 65 MPH anyways, so the speed limits are a bit lower anyways, I would spend most of my time at 45-50 MPH max anyways.

So I guess the 3.91s would be the best choice for what I want (acceleration over speed) If anything I can add a bit more tire on the rear, which I may do anyways because the new rear end can accommodate a bigger tire. Because currently the car isnt the funnest to drive, the 2.76s suck the life outa it with my engine combo, just dont want the 3.55s to only be a marginal increase in performance.
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How do you plan to put more than 28" tall tires on it?
 
I've run both and personally for a street car I really like 3.55's but if you are rarely going to go over 60 mph and do want to race it the 3.91's will work well for you. I ran 3.91's for years, used to drive my 340 Dart to the track (about a 60 mile trip one way) and had good luck with it. I have also run 4.56 gears on the street and well that's a tad much for highway driving LOL.
 
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