|
By salf100 - 14 Years Ago
|
|
What would be the best differential gearing in my 9" with stock 3 speed. Im looking for a better all around gear that will allow me to cruise at 70mph and not cause the 292 to scream. My current gears are 3:89 and Im running a 15" tire.
|
|
By charliemccraney - 14 Years Ago
|
That's tough to say. The absolute best option is an overdrive transmission.
Obviously, any gear numerically lower than 3.89 will be a benefit, but it will take a 2.75 to get your RPM in the 2500 neighborhood. If you do that, forget about acceleration and forget about using it even remotely as a truck.
The good news is that many truck 9" center sections will swap right over, to about 1976, so this doesn't have to be expensive.
|
|
By The Master Cylinder - 14 Years Ago
|
Here is a formula to help you figure out what Gear Ratio to use for a given RPM, Tire Diameter and MPG.
Gear ratio =rpm x tire diameter/Mph x 336
Example 3000 X 27 divided by 65 X 336... 81000/21840=3.70:1
Hope that helps
|
|
By The Master Cylinder - 14 Years Ago
|
Should have used Charlie's number, to get to 2500 RPM, everything else the same in my example, you need 3.09:1 ratio.
Plug in your own numbers, I don't know how tall your tires are. I'm running 3.55's with a 27" tall tire and it seems be a pretty good compromise for town and highway.
|
|
By Oldmics - 14 Years Ago
|
|
Salf If interested I have a 9" pumpkin that is either a 322 or a 291 that I could swap out to you. I"m looking for steeper gearing for my situation. My unit is a 31 spline device. Lemmie know Oldmics@Yahoo.com
|
|
By charliemccraney - 14 Years Ago
|
|
Oldmics (3/27/2011)
My unit is a 31 spline device.
A stock '64 will be 28 spline. Check the axle tag to find out just what you have; it could be a 31 spline by now.
|
|
By salf100 - 14 Years Ago
|
|
I have 6.70X15 tires (27"). Ill look and see what size splines I have, 28 sounds right. And swapping the trans is not an option right now. I will trade anyone who wants to. I was thinking 3.25ish should be good. Better than what I have! Master, 3.55 work good for you? what vehicle are you running them in? The truck is lowered and not used to haul anything. Would a stock 4 speed be ideal? And what would a used 4 speed cost?
|
|
By pintoplumber - 14 Years Ago
|
|
I now have a 4.27 rear which I really like. When I'm in overdrive the effective gearing is 3.00. I can now start out in overdrive 1st, but I haven't tried it on a steep hill. I think 3.25 would be better. With the old rear 3.92 and overdrive the effective gearing was 2.43. I couldn't start out in overdrive 1st unless I was pointed downhill.
|
|
By speedpro56 - 14 Years Ago
|
|
I have a 3.10 9" I would swap. I in Asheville, N.C.
|
|
By speedpro56 - 14 Years Ago
|
|
I forgot to add 28 spline.
|
|
By charliemccraney - 14 Years Ago
|
|
salf100 (3/28/2011)
I have 6.70X15 tires (27"). Would a stock 4 speed be ideal?
It won't be ideal, but it will get back some of your truckability with the granny 1st gear and a tall rear gear. Keep in mind that with the stock 4 speed, 2 - 4 is the same as your current 1 - 3. But really, if you're considering this, save money and do a 5 speed and don't touch the rear gears. You'll have pretty much the same amount of work involved, actually it'll probably be easier because then you don't have to mess with the rear gears.
Stock 4 speed, you'll need to:
change the bellhousing
cut a hole for the shifter and possibly even clearance the floor for the transmission.
maybe change the drive shaft length
possibly change the clutch while you're there.
5 speed:
Different, modified bellhousing
cut a hole for the shifter
shorten the drive shaft
replace at least a couple clutch components.
You see, it's very much the same. and when compared to the 3speed, a 5 speed will give you a first gear which is like going up numerically by 1 or 2 ratios in the rear, a 4th gear to play with which makes it more fun and is sometimes more practical on slower roads, and a 5th gear that gets you better mileage on the highway. The 5 speed probably will cost a little more but you also get a whole lot more.
|
|
By The Master Cylinder - 14 Years Ago
|
salf100,
The 3.55 work good for me but if you do a lot of highway driving you may opt for the 3.25's. I do mostly town driving. My 3.55's are in a '55 Ranchwagon with C-4 trans and 27" tall tires. By rearranging the formula you can try different MPH and Gear ratios to check RPM; mph x gear ratio x 336 / tire diameter=rpm.
Example; 70 x 3.25 x 336 / 27 = ~2800 RPM or lower (numerically) 70 x 3.00 x 336 / 27 = ~ 2600 RPM. Just to confuse things, you could also lower the RPM by putting on a taller tire. (I know, you didn't ask that but thought I would throw it out there, anyway, haha).
Clear as mud, right??
|
|
By Butch Lawson - 14 Years Ago
|
|
Or, you can go to: www.angelfire.com/fl/procrastination/rear/html and play around with the numbers.
|
|
By speedpro56 - 14 Years Ago
|
|
with a 3.00:1 27" TH (tire hight) going 60 mph you would be turning 2240 rpms. with 28" going 60 mph you would be turning 2160 rpms. with a 4-speed and granny 1st you could get buy in this neighborhood easy enough, and tire height will help some.
|
|
By slumlord444 - 14 Years Ago
|
|
Ford's standard rear end with a 3 speed back in the mid to late 50's was a 3.55. A 3.00 will be hard on the clutch from a stop sign while better on the highway. I have a 3.55 in my T-Bird that I would gladly trade for a good 3.89. Drag racing is more my game.
|