Differential Ratio

[vping]

I have three rears (no laughing). One is from a 1969 AH Sprite. One is from a 1971 MG Midget & the other is from a 1972 MG Midget. How can I tell what the ratio of each is?
Is it stamped on the housing?

 

[ChrisS]

The teeth count will give you the ratio. It's stamped on / around the gears, but I forget were. They should be 4.22:1 (9x38 teeth), or 3.9:1 (10x39), or 3.73:1 (11x41). The early rears were 4.22, I'm not sure what was installed '69, but the '71 and '72 should be 3.9. The rubber bumber cars had the 3.73.

 

[jollyroger]

They are stamped in two places. You should find a stamped number on the top of the pumpkin on the right (passenger) side just below the mounting flange. It should be something like 9/38(4.22) or 10/39(3.9) or some other combination. Dividing those will get you the gear ratio. But...there is always a but, If someone has changed the gears it could be different. Soooo, look at the the ring gear, somewhere on it you will find the same numbers stamped on the flat outside portion of the ring gear. Divid the small one into the big un and there you have it.

 

[Matthew E. Herd]

Actually, so far as I can tell, only the latest RB cars were 3.73. Mine's a 3.9, and it's a 76. I think it was a '79 vintage effort to reduce highway revs a bit (somewhat hopeless at that point).

 

[SilentUnicorn]

i had a 373, replaced it with a 39 best thing i ever did for that 1500. made a huge difference in standing start at a lite.

mark

 

[jollyroger]

Mark,
You still have the 3.7 diff? I am kinda looking for one to experiment with at the track.

 

[SilentUnicorn]

Lol i didnt mention why i had to change it did I?? Um lets just say it failed to launch? the spider gears blew up. take a good look at the attach.


mark

 

[jollyroger]

Did it hurt the ring and pinion? If not, do you want them?

 

[spritenut]

69 was the change year fro 4.22 to 3.9. the others are 3.9 for sure. There are numbers as others have said or just turn the input and count the revs. 4 and 1/4 = 4.22, almost 4 = 3.9 and 3 and 3/4 is a 3.77 or close enough by eye and hand turning.

 

[vping]

I think the 69 Sprite might be a 4.22. This is soley based on the vin #. Says the car had a close ratio and a higher compression engine but I am not sure. I will count & see.

 

[GB1]

Very Good info

I believe that Midget\Sprites through 1500 were 4.2, then 3.9 with the intro of the 1500 and for the last few years (2?)of manufacture were 3.7

So probably all three of your diff's are 4.2

please correct me if I am wrong

 

[jollyroger]

I believe that all 1275 powered cars came with 3.9 diffs. that doesn't mean that they now all have them. I run 4.2 units in a number of my 1275 cars. Mine seem to like to run at 4000 to 4500 rpms at speed. 70 plus mph. However I could be wrong?

 

[GB1]

All 1275's that I have owned have been 4.2. The best way to check is by the stampings accurately named by above.

The RPM to MPH also depends on the tire sizes, it seems from your comparison that you are running a lower profile then OE. With OE diameter the MPH will be roughly twice the RPM's on a 4.22.

Please use the stampings or counting gears for an accurate comparison for the ratio's quoted above. Comparisons of RPM's/MPH's are not as accurate.

 

[jollyroger]

I agree, check the gearing from the ring gear if you really want to know. A quick way is to turn the drive shaft and count the turns of the shaft it takes to turn the rear wheel one rotation. If it is an open diff. while the axle is off the ground hold one wheel from turning and turn the driveshaft. count the turns of the driveshaft it takes to rotate the wheel one time then multiply that by two. That will give you the ratio. Are we confused yet. I could explain it further but my eyes would cross.

 

[ChrisS]

The data I have says that the ratio was changed from 4.22 to 3.9 in Dec. ’68 (HAN9-77591 / GAN4-66226) then again in Aug. ’77 (GAN6-200001) from 3.9 to 3.7. (Source Terry Horler).

Chris

 

[SilentUnicorn]

jolly , i'll have to go and dig it out and take a look at it. i'll let ya know


mark