I pulled the 8 3/4 center section on my wife's 1971 R/T Challenger today.

I had previously done the spin test and was pretty sure it was 4.10 ratio.

Well I guess I was close.
In looking at the stampings it appears to be 4.11 ratio.
I thought 8 3/4 rear ends only had 4.10?

The center section is a 489 with cone type Sure Grip.

Am I missing something in checking the stampings?

That be 4.10.

4.1:1

correct.........................

So that would be the tooth count 41 and 10?

Proper would be


(4.1:1)

4.1:1,,,,,,,,,,,,,,,,,,,,,,,,,,,,,errr, 4.11:1 is splitin' hairs. it is only 1 one hundrenth?

I would think dividing the teeth numbers would give an OCD person an actual number (not that I could relate to that at all!)

I have a vague memory that some old 4WD trucks had (very) slightly different ratios like that 4.10 and 4.11 in the front vs. the rear diffs, so that the front always pulled the rear rather than being pushed. Anyone remember this? Or is it another urban legend...

4 x 4 Dodge trucks had 4.10 gears in the rear and 4.09 gears in the front.

Count the teeth, my 4.56's are actually 4.57's when you calculate it.

All the manufacturers do that, the bigest spread I have seen is in Fords where the 1/2 tons used 3.50 in the rear with the 9 inch and 3.55 in the Dana 44 front..

The reason for the different ratios goes way back to the drawing boards, in most cases 50 years ago or so. Different rear ends had different stock ratios available. For example, the Dana 60 comes in 4.10, 41 to 10 teeth. The 83/4 was 4.1 also. Plenty of others were 4.11, 37 to 9 teeth. But they were within 0.24 percent of each other, so could be used on the same vehicle with no problems. The Dana 44 4.09 ratio was 45 to 11 teeth. Also within range.

Keep in mind that manufacturer's directions for those 4x4 vehicles specified not to be operated on dry pavement. The primary reason for this is the different radii between front and back when turning. But, the small difference of gear ratios would be taken care of as well.

The idea of mismatched gear ratios for one axle pulling more than the other is incorrect and is the result of people trying to explain the idea while having no idea of how manufacturing works.

There are numerous ratio clusters that have been used together. For example, 3.21s and 3.23s, 3.08 and 3.07, 3.90, 3.91 and 3.92.

Many gear designers tried for gear ratios that weren't even to the tenths place. This was to keep the same pinion tooth from contacting the same ring gear tooth time after time. Of all the companies building rear ends, Ford didn't seem to have a problem with 3.50s, 4.10s, etc. Then there was the trade-off between noise and tooth strength. Gear designers think about all sorts of things.

R.

Doc I am curious, if 37/9 teeth give 4.11, what teeth count did Ma use to get 4.09?

I know you meant Dr. Diff but Dogdays covered that in his post just above

by God he did, I gotta start paying more attention to my buddy Dog

Its more about it being possibly "chevy like" to some of the mopars or no cars on here.....