Flyinbrian72duster
Dart and duster
Gonna change 7 1/4 2:76 to 8 3/4 it' a stock 318 4bbl what' a good ratio was thinkn 3:55 anyone know from experience
Gear ratio
- Thread starter Flyinbrian72duster
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AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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3.55s and a 2800TC are a fine combo for an otherwise stock teener with a 4bbl and headers.... or at least free-flowing dual exhaust. With a stock cam, headers are optional.
Actually 3.23s are pretty good too, and if hiway traveling is on the table then 3.23s and a 2400TC might be a better choice.
Also involved is the tire size.
The tallest tire you can likely put on an A-body is a 28, and a common size is a 25.5, but there are 24s available. There is about 10% difference in each of these, as it affects gearing; same as the gearings from 2.94 to 3.91, skipping 3.73s.
So a tall 28 and 3.55s is close to the same as a 25.5 with 3.23s, is about the same as a short 24 and 2.94s. So be careful.
Here's he formula,for effective gearing
and how the math works out;
rear gear x 12, divided by 1/2 the tire diameter
3.55 x 12/14.00 = 3.04
3.23 x 12/12.75 = 3.04 zero difference
2.94 x 12/12.00*= 2.94 3.3% difference to 3.55s
2.76 x 12/12.00* = 2.76 10% difference to 3.55s
(*)24s are about the smallest 14s
The difference from 2.76s to 3.55s with the same tire and stall, however, is 3.55/2.76 = plus 28.6%. So you can expect your 318 to feel 28.6% stronger with the new gear.
As to stall, if the current 1700TC lets the engine rev to 150 ftlbs, and the 2800 to 250, then this is 250/150= plus 66.6%
Combining them in first gear, you get
current; 150 x 2.45 x 2.76 x 12/12.75 = 955 ftlbs to the road.
new;.... 250 x 2.45 x 3.55 x 12/12.75 = 2046 ftlbs to the road, and that new rear better have a Limited-Slip in it!. This is an increase of 2046/955= plus 114%, or just over double. 2046 is enough to spin even 28s..... both of them.
Hang on; I'm not saying your teener can make 250 ftlbs at 2800, or that it can only make 150 at 1700. But it's irrelevant , the result is the same; there is still gonna be tirespin.
I know.... because mine did, and it was a 17 year old, tired low-compression teener, out of a 1973 Swinger 4-door, transplanted into my 1968 Barracuda, with; a 904(2.45 low) one winter, a 998(2.74 low) another, a 3.09 manual trans a third, and fourth, and back to the 998 for it's final winter. Then I put it into my sons D100, with 3.23s, and he drove the crap out of it, too, for about 6 or 8 more years. Then he traded that back to me for a 95 Sunfire. So I've had that engine since about 1980. And AFAIK it's still on it's 1973 rings,lol.
The biggest performance increase will come from the higher stall TCs. A 2400 is a great upgrade with any rear. The 2800 will really wake your teener up; it is super big fun with 3.55s, which can let you hit 60mph pretty much revved right out in second gear.
If you still have the stock cam in there, I highly recommend to leave it in there, and try the 3.55/2800........ first.
If you have a bigger cam and found the bottom end went soft on you, then the only cures are; more compression, more stall, more gear, or less weight; in the most effective order.
Oh yeah, put some better valve springs on it cuz if you start wringing it out, the lifters will pump up shortly after 4800. 360 2bbl springs will take her to 5500 OK.
Actually 3.23s are pretty good too, and if hiway traveling is on the table then 3.23s and a 2400TC might be a better choice.
Also involved is the tire size.
The tallest tire you can likely put on an A-body is a 28, and a common size is a 25.5, but there are 24s available. There is about 10% difference in each of these, as it affects gearing; same as the gearings from 2.94 to 3.91, skipping 3.73s.
So a tall 28 and 3.55s is close to the same as a 25.5 with 3.23s, is about the same as a short 24 and 2.94s. So be careful.
Here's he formula,for effective gearing
and how the math works out;
rear gear x 12, divided by 1/2 the tire diameter
3.55 x 12/14.00 = 3.04
3.23 x 12/12.75 = 3.04 zero difference
2.94 x 12/12.00*= 2.94 3.3% difference to 3.55s
2.76 x 12/12.00* = 2.76 10% difference to 3.55s
(*)24s are about the smallest 14s
The difference from 2.76s to 3.55s with the same tire and stall, however, is 3.55/2.76 = plus 28.6%. So you can expect your 318 to feel 28.6% stronger with the new gear.
As to stall, if the current 1700TC lets the engine rev to 150 ftlbs, and the 2800 to 250, then this is 250/150= plus 66.6%
Combining them in first gear, you get
current; 150 x 2.45 x 2.76 x 12/12.75 = 955 ftlbs to the road.
new;.... 250 x 2.45 x 3.55 x 12/12.75 = 2046 ftlbs to the road, and that new rear better have a Limited-Slip in it!. This is an increase of 2046/955= plus 114%, or just over double. 2046 is enough to spin even 28s..... both of them.
Hang on; I'm not saying your teener can make 250 ftlbs at 2800, or that it can only make 150 at 1700. But it's irrelevant , the result is the same; there is still gonna be tirespin.
I know.... because mine did, and it was a 17 year old, tired low-compression teener, out of a 1973 Swinger 4-door, transplanted into my 1968 Barracuda, with; a 904(2.45 low) one winter, a 998(2.74 low) another, a 3.09 manual trans a third, and fourth, and back to the 998 for it's final winter. Then I put it into my sons D100, with 3.23s, and he drove the crap out of it, too, for about 6 or 8 more years. Then he traded that back to me for a 95 Sunfire. So I've had that engine since about 1980. And AFAIK it's still on it's 1973 rings,lol.
The biggest performance increase will come from the higher stall TCs. A 2400 is a great upgrade with any rear. The 2800 will really wake your teener up; it is super big fun with 3.55s, which can let you hit 60mph pretty much revved right out in second gear.
If you still have the stock cam in there, I highly recommend to leave it in there, and try the 3.55/2800........ first.
If you have a bigger cam and found the bottom end went soft on you, then the only cures are; more compression, more stall, more gear, or less weight; in the most effective order.
Oh yeah, put some better valve springs on it cuz if you start wringing it out, the lifters will pump up shortly after 4800. 360 2bbl springs will take her to 5500 OK.
Last edited:
Charrlie_S
Well-Known Member
What transmission? What type of driving? Mostly around town, mostly hiway, Mostly flat roads, or hilly/mountain, easy cruisin, or pedal to the metal smoking the tires? And as asked what tire size?
PS: Also does fuel mileage matter?
QuickDart360
Well-Known Member
3.91's
abodyjoe
Well-Known Member
Stick 318.
2.94 or 3.23 gear would be grea and the direction I’d go. But I like driving my cars distances on the highway at 80 or so mph.
2.94 or 3.23 gear would be grea and the direction I’d go. But I like driving my cars distances on the highway at 80 or so mph.
Flyinbrian72duster
Dart and duster
904 just cruising around very little highway flat roads but I do like the occasional burn out tires are 195/70/14 was gonna put a little meatier tire after swap fuel mileage don' matter
abodyjoe
Well-Known Member
Then put 4:30’s in it. 
leblanc
member 2883
I have 3.55's and a 904 trans in my Dart Sport, love them, they will set you back in your seat a lot more than what your used to. If you wish you could run an 8 1/4 rear end and likely save yourself a few dollars (unless you can get a 8 3/4 for free) the 8 1/4's are more plentyfull. The 8 1/4 can handle up to 300-350 HP which a stock 318 does not have yet. I believe 391's or higher are going to cut too much in your gas bill and are kind of a high gear for a stock motor.
abodyjoe
Well-Known Member
8 1/4 are getting stupid in price these days too.
Almost worth getting say a cbody housing cut down to abody size.
Almost worth getting say a cbody housing cut down to abody size.
My car is '68 cuda, 4-sp, near stock G70x14 redlines. I love my 3.55 gears. A good compromise street gear. A little quicker stop lite to stop lite and a reasonable RPM on the freeway-3200.
Charrlie_S
Well-Known Member
Then I would say 3.55 or 3.73 with a bigger tire
456's plenty of power and you can still can go 130 +
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