A "tall" (lower numerically) rear end gives you higher top speed but less acceleration. A "low" (higher numerically) rear end gives you lower top speed but greater acceleration. Confusing, I learned to remember based on top speed.... tall/high, low/slow.

The mystery is unraveling very quickly guys, thanks. Some more "do I have this right?"'s.....
I'm assuming that the "closer to a 1:1 ratio" that Gary mentioned, and Bob and Lynn were refering to: That is the ratio of the differential's gear to the tranny's 1st gear?
Also...the ring gear as stated can have a different number of teeth, while the pinion gear remains constant at, in our case, 9 teeth. I'm trying to figure out how a ring gear can have a differing quantity of teeth, so I'm concluding that the difference has to come in in the dia of the ring gear (or the spacing of the teeth?), with a corresponding  adjustment on the pinion gear?...thus the matched sets.
Another way of asking...I'm assuming that all 9 tooth pinion gears are not the same, as they have to have an adjusted dia (or tooth spacing) to match the different ring gears....so a 9 tooth pinion gear for a 3:56 is different than a 9 tooth pinion gear for a 4:11
LOL..a third way of asking...what is the variable that allows the different number of teeth on a ring gear?

Quote from: RICH MUISE on 2015-06-11 07:14
The mystery is unraveling very quickly guys, thanks. Some more "do I have this right?"'s.....
I'm assuming that the "closer to a 1:1 ratio" that Gary mentioned, and Bob and Lynn were refering to: That is the ratio of the differential's gear to the tranny's 1st gear?
Also...the ring gear as stated can have a different number of teeth, while the pinion gear remains constant at, in our case, 9 teeth. I'm trying to figure out how a ring gear can have a differing quantity of teeth, so I'm concluding that the difference has to come in in the dia of the ring gear (or the spacing of the teeth?), with a corresponding  adjustment on the pinion gear?...thus the matched sets.
Another way of asking...I'm assuming that all 9 tooth pinion gears are not the same, as they have to have an adjusted dia (or tooth spacing) to match the different ring gears....so a 9 tooth pinion gear for a 3:56 is different than a 9 tooth pinion gear for a 4:11
LOL..a third way of asking...what is the variable that allows the different number of teeth on a ring gear?

The 1:1 ratio was a hypothetical rear end ratio. The closer you get to this number the slower the engine turns.

Earlier, I was talking about the 1st gear ratios. OVERALL GEAR RATIOS should be considered in selecting a rear end ratio. The over gear ratios are determined by multiplying the rear end ratio times the transmission gear ratio in any given gear. Overall 1st gear ratios between 10 to 1 and 11 to 1 should be about right for a 4000 lb car. If your transmission 1st gear ratio is 2.84 and you have a 3.55 rear end that gives you (2.84 x 3.55) 10.08 overall in first, pretty good. My car has a wide ratio toploader (2.72) and a 3.89 rearend for an overall of 10.6 for comparison. A close ratio muncie (2.20) and a 4.56 rearend is only 10.03 overall in first gear! The difference in the overall gear ratio in high gear between these examples is very large because your transmission has overdrive. 3.55 x .7= 2.48 for your car, 3.89 for mine, and 4.56 for the hypothetical GM. The GM car is turning 4127 rpm at 70, mine 3520 rpm at 70, yours 2250 rpm at 70 (assuming 26" diameter tires).


The shape of the teeth between the ring and the pinion must match in such a way to give a specific contact pattern. They are sold in sets to get close, but a differential has to be "set up".... both gears must be shimmed to get the contact correct. When the number of teeth are changed on the ring gear, the shape of each tooth changes. I am not positive but I think the pinion gear shape (profile) can stay the same. Gear teeth actually "roll" together as they make contact, making the face of the teeth have a very specific shape. Besides that, modern rear differentials (including 57s) have hypoid gears, the teeth themselves are in a spiral pattern. This allows for a larger contact pattern, or a stronger gearset. "Lower" (higher numerically) gearsets for a fixed pinion size can transfer more torque without breaking than a "tall" gearset can.

Readers beware, the information above was recovered from a senior memory ....

Great explaination, Lynn. Thanks. 3.55's it'll be then for me...as I said before, I'm not building a racer. Also, as Bill said the donor MarkViii had a 3.55, and though no speed demon off the line, they were pretty responsive, and my car is hundreds of pounds lighter than a Mark.
Now, just to clarify one more thing....we have been using both periods (3.55), which I think is correct, and colons (3:55), which I'm thinking is incorrect, and also the root cause of my past confusion because the colon implies there is a comparison between the "3" and the "55"...correct??

3:55 is a typo. 3.55 is an excellent choice!

OK..cool. I was afraid you were going to disagree and send my understanding back to square one!! To go one step further with that question...technically then, when we speak of a 3.55 ratio, the colon would come in with the full description...3.55:1 ?

3.55:1 is the correct technical term, sir. Originally, I thought your first gear was 2.4:1. I read the info wrong and looked at the ratios for an AOD. That would have made your overall gear ratio in first 8.52:1, to me that would have not been "low" enough... GIGO! (Garbage in, garbage out). I actually like the gear spacing you have it seems ideal in first and forth, much better than mine. My car has a top speed of about 100 mph, at 75 it is really humming. I plan on addressing that at some point... no substitute for overdrive!

Did a little research...my base'95 MKVIII came equipped with a 3.08 differential. (the  LSC models had a 3.27 for a quicker off-the-line, mine was not an LSC). So I'm gathering from all the info, if I choose the 3.55 for my 9", it should be substantially quicker off the line than a Mark VIII, not even considering the substantial weight advantage. I guess all of this is confirming my original thoughts with my engine/drivetrain choice....reasonably quick, dependable, and economical.

I do apologize for the 3.55 error, got my book out and saw that the 3.27 was in fact the correct ratio, also for the colons, just lazy on my part.....

A little yard searching and an Explorer rear with 3.55s and a T-Lock could be found and narrowed for cheap.  Many of the Explorers had T-Locks if AWD.

Guys:

I, too apologize for the many colon errors I made.  Rich, I think you will be very happy with a gear around 3.5 as in the original Ford 9" rear ends or a 3.55 as in the modern 8.8 Ford rear ends.  The fact that drag racing does not play a role in your thoughts should also not bother you in the type (8.8 or 9") of rear end you select.  Also, either type of rear end has massive after market support.

Bob
57 AGIN

OK...now I know you guys are out to confuse me, LOL. Bob as I undewrstand it, the '57's because of the 9 tooth pinions also had the 3.55 as an option (32/9=3.5555), not just the newer variety.

like i said ford used a 3.56 ratio in the 9 inch diff till 1968.in 1969 ford went to a 3.50 ratio for the 9 inch diff.

3.55 or 3.56, depends on whose rounding off. No reason why you can't have a 10 pinion gear in a 9" rear end.