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Change the rear end.

blue69runner

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So I am going to change from 3:55 to a 3:23 ratio on my dif. So on the road want to lower my rpm's. Hope to get a little better fuel mileage will this help or should I go to a different combo. Thanks for any input. 383 with four speed in a 69 roadrunner.
 
I wouldn't bother. The ratio change is insignificant in my opinion. (I haven't calculated it, but I would be surprised if it's much over 5%).
I have a factory 4.10 Dana car, and would like less rpm too, but until 3.23s came out for the Dana, I thought that the previous highest gear, 3.54, would not change enough to make the swap worth while.
If you really want a change..... I would go to a 2.xx gear..... and be ready for a difficult to drive, slow, roadrunner.
 
Went from 3:91's to 3:23's major drop in RPM at speed. Will definitely help on fuel consumption. But as posted above probably not much difference in your ratio.
 
I agree that 3.55 to 3.23 isn't enough. It's your call. I went from 3.55's to 3.91's and now 2.76's. I don't have overdrive. 2.76's are slow off the line, but a real cruiser on the highway. Depends on what you want.
 
Just switched from a 3:23 peg legger to a 3;55 sure grip. Much better out of the hole.
 
It will drop your rpm at 60 by 200-300. To me a car with 3.23's and 28" tall tires is just fine at 70 mph, whereas the same car with 3.55's is a 60 mph car.
 
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To me a car with 3.23's and 28" tall tires is just fine at 70 mph,
I have 3.23 and a 30'' tire and agree with you.
blue69runner, how tall are your rear tires ?
One of my all time ratios are the 2.94 if you can find them. I have run that in several cars and play with rear tire height to dial in.
MY 2 cents.
 
2.94 is what I have in my Bee, but your clutch won't love you starting on hills and the Cops won't like you on the highway....:D
beerestoration2015-2016 387.JPG
 
If you need rear tires go to a tall tire. 3.55 to 3.23 isn't much of a drop. I don't know how many miles you drive or your finances but the savings on gas the payback would take a long time. You would lose some of the fun factor. Wear and tare on your engine long payback. Ran 3.55's with 26.25" tires 3000/3100rpm @70. Drove CT to Niagara Falls at mostly at 70mph 14 mpg. Any faster the front carb on the Hemi starts to open so that the point mpg goes south. A noisy red car is a target.
 
My bet is you won't see much MPG improvement in general from a 3.55 to 3.23. It would be a lot simpler to use some taller tires that fit your wheel wells.
 
I have 3.23 and a 30'' tire and agree with you.
blue69runner, how tall are your rear tires ?
One of my all time ratios are the 2.94 if you can find them. I have run that in several cars and play with rear tire height to dial in.
MY 2 cents.
28 inch on rear of car. I am getting about 7.3 miles to the gallon right now. Would like a little better millage LOL. Like was stated would like to lower the RPM's some also. The original gearing was 7:27 way to much the other way but would bet it would run up to 120 mile per hour. Know it would drag of like a lead sled from the get go. Thanks for the info. Some thing to think about.
 
Thanks all for the info. Maybe if I could keep my foot out of the carb it would get better fuel millage. But it is not easy to do that. Ho well maybe fuel will come back down some and can enjoying filling her up. Blue.
 
The gearing change you propose is around nine percent. Going from a 28"tall tire, to a 30" tall tire is just over seven percent.
And a tuneup, and maybe a jet change should help that 7 1/2 mpg, IF you can keep your foot out of it. (My 440, Sixpack with 4.10s did better than 7 1/2.)
 
That is good info. A gear vendor my be in the future. I like the way the car performs. I am running 80 jet's in the 650 cfm Carb right now. It did not like the 76 jetting that was in the carb. Stumbled and stuttered. Would not idle. So will see. Don't drive it that much for now and if fuel prices keep going up may just have to drive on special event's. Thanks 33 IMP. Oh just to say when watching Orange county chopper's. They built an all E85 bike and had to double the jetting size to run Ethanol. My motor guy's said that with the ethanol mix in our fuel is the cause for consuming more fuel to run these big block motor's. Even with real gas back in the day they did not get that good of mileage either. But fuel was a much better grade and had lead in it so our motor ran better and lasted longer. Well I am from the era where their was white gas and red lead fuel. Those day's are long gone.
 
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It just sound's so good to here that Four barrel kick in when doing 55 mph and get up to 70 quick. LOL. :thumbsup:
 
My 70 383 Challenger went from 12 mpg to 16 going from 3.23's to 2.94's. That was a nice surprise! Was expecting 14. Gotta turn a big block kinda slow to see a decent increase in mileage imo. If you had a small block, you would be a good increase in mph in town staying with the 3.55's. Heck, my /6 Dart got 20 in town with 3.91's even with me not being nice to it but it sucked the gas down at 60 on the highway.
 
It's really simple math. It will drop your RPMs 10%. In my experience you do not want to go to a higher ratio than 3.23 with a clutch and regular transmission ratios. 3.55 are not that great of a gear, not good at anything.
 
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