Diff Gear Ratio

[John Moore]

I've heard that the gear ratio is supposed to be stamped somewhere on the top of the differential. Where should I look? And what would the markings be?

My bugeye came with lots of later spridget stuff including a 1275. I wonder if the DPO also left me with a 3.9 or 3.7. That would be nice.

 

[bill_young]

The numbers are stamped on the edge of the ring gear, but that wouldn't be visible unless you remove the diff. Probably easiest to figure out by using a gear ratio table such as this and plugging in your data as far as tire size, speed, and rpm. That would calculate the diff ratio in 4th gear easily. https://www.f-body.org/gears/

 

[Gray_Cat]

John:
The sure way to determine the gear ration is to find the numbers stamped on the "ring" (larger) gear and devide the large number by the samll number and you will know exactly what you have. Just rotate the gear until you see the numbers, easy to see. The numbers are also stamped on the casing the pumpkin is in. Only problem is, someone could have changed out the gears to another ratio so you shouldn't rely on those numbers alone.
Roy

 

[John Moore]

Yeah, right now I haven't disassembled it yet. As Roy mentioned I thought it was stamped on the outside, but you're right that could have been changed!

 

[Hap Waldrop]

Yeah, right now I haven't disassembled it yet. As Roy mentioned I thought it was stamped on the outside, but you're right that could have been changed!

I always heard they were stamped on the outside from various case, bet i had my hand on 50 Spridget diffs, all sorts of ratio, never saw one stamped on the outside.

Word of caution, the 3.70 is a good highway gear but a crappy mountain gear, and good way to burn up clutches on hills and mountains. I know Ray is talking about doing a 3.70 with a Datsun 5spd, but if he does any mountain work in this car, he'll probably change that. David "Gundy" who post here, says his 3.70 with the rib cage 4spd suxs in the mountain roads.

 

[Gundy]

Yeah, right now I haven't disassembled it yet. As Roy mentioned I thought it was stamped on the outside, but you're right that could have been changed!

I always heard they were stamped on the outside from various case, bet i had my hand on 50 Spridget diffs, all sorts of ratio, never saw one stamped on the outside.

Word of caution, the 3.70 is a good highway gear but a crappy mountain gear, and good way to burn up clutches on hills and mountains. I know Ray is talking about doing a 3.70 with a Datsun 5spd, but if he does any mountain work in this car, he'll probably change that. David "Gundy" who post here, says his 3.70 with the rib cage 4spd suxs in the mountain roads.

Worse yet is I'm having to feather the..gasp...clutch to get up my driveway. I have to stop and wait on the gate to open
then go uphill from the apron. That or rev the girl to about 4K and drop the clutch.Not good. I suspect some of my problems
also stem from the lightened drivetrain and skewed to top end
engine performance. That sucker has ZERO torque....but
it wails after about 4K up to 7K+.
I'd probably keep the 3.7 back there if we didn't have the MGB for longer hauls on a highway. As it is now the most I have to drive to have fun is about 15 minutes...sooo..
4.22 goes in tomorrow. Heck nothing wrong with 6k highway
revs anyway. With the straight through exhaust and straight cut tranny gears I'm a little deaf after a run however.

 

[regularman]

Yeah, right now I haven't disassembled it yet. As Roy mentioned I thought it was stamped on the outside, but you're right that could have been changed!

I always heard they were stamped on the outside from various case, bet i had my hand on 50 Spridget diffs, all sorts of ratio, never saw one stamped on the outside.

Word of caution, the 3.70 is a good highway gear but a crappy mountain gear, and good way to burn up clutches on hills and mountains. I know Ray is talking about doing a 3.70 with a Datsun 5spd, but if he does any mountain work in this car, he'll probably change that. David "Gundy" who post here, says his 3.70 with the rib cage 4spd suxs in the mountain roads.

Just because its a 5 speed doesn't mean that he has to drive in OD. 3rd in mine is about where 4th used to be with my 3.90 and I have two more gears if I need them. 4th is the gear I needed for the roads around here and can stay in it most of the way up to mount pisgah. OD is great on the interstate. It is so much quieter in the cockpit with that gear on the highway.

 

[Trevor Jessie]

There is a flange on the diff nose where the ratio is stamped. Of my three diffs, it is legible on one. On the other one it is obvious that it was there, but no longer legible. On the third there is no trace of a stamping. It is stamped on the top side of that flange (virtually unreadable if the rear end is still under the car)

My car with a 3.9 pulls like a freight train in 1st and 2nd gear. I can not imagine that switching to a 3.7 would make it a pig.

 

[aeronca65t]

I raced my car with a 3.7 with no problems for years. My car has a pathetic 1500 engine that is basically stock and far less powerful than a decent 1275. I just switched to a 3.9 because I finally fried the 3.7

With the 3.7 my car would smoke the 185/60-13s at hillclimb starts.
(well it'll smoke <span style="text-decoration: underline">one</span>....it's an open diff)

I had a 3.7 in a 1275 street Spridget years ago...it was no problem.

 

[Sarastro]

Here's a simpler way to check the ratio.

1. Jack up one rear wheel & block the other. Keep the brake off and put the transmission in neutral.
2. Turn the wheel exactly one turn.
3. Count the number of turns of the drive shaft.
4. There will be a fractional turn of the drive shaft, which you will have to estimate. Starting with chalk marks helps.
5. Multiply the no. of drive-shaft turns by two.

It's easy to tell whether you have a 3.9 or 4.2 by this method. To distinguish between a 3.7 and 3.9, you may have to be fairly precise in estimating the fractional turn: chalk the drive-shaft flange, measure it with a tape, that kind of thing.

Still a lot easier (and less messy!) than dropping the differential.

 

[Gundy]

Guess I'm just used to MBs, GTOs and Land Cruisers. Torque
monsters all.
Don't get me wrong, my BE runs great and it ain't a pig
by any means, it's just that a 4.22 is a better choice for my conditions and set up. Some hairpins I encounter really take the BE out of it's power range in 2nd and double clutching to
first isn't something I'd like to try. I think the lower
rear gear will help me maintain my powerband a bit better.
There's no denying it'll be quicker. Simple math. The local
fun rides will have few long straights and most of the "work" will be under 50mph. No 4th gear stuff. It still pulls under
2K rpm but it REALLY pulls over 4K.
The wife's MGB is smooth and torquey and has no problem
with the driveway entrance but I can promise you my BE would smoke her MGB even with the 3.7 in the Sprite.
I guess I could get a tranny with a syncro 1st, say a Datsun,
but I want to keep the Sprite all Brit.
If the 4.2 doesn't satisfy my needs I may go 4.56 or something.
I'm not knocking the 3.7..ol Hap dragged me into this....
it's a GREAT rear gear and very desirable. It just ain't for
me given my requirements. I LOVED the 3.7 cruising down the highway and it gave me a ridiculous top end speed. It would still accelerate sharply at 80mph. Just different requirements now. I will NOT be selling the 3.7. If needs change it'll go right back in. :thumbsup:

 

[nomad]

I have several diff's around here and I have found the ratio stamped on all but one. Scrape the grease away where the diff bolts into the axle housing at about the 10 O,clock and 2 O,clock position. Stamped with a die stamp.
KA.

 

[John Moore]

I have several diff's around here and I have found the ratio stamped on all but one. Scrape the grease away where the diff bolts into the axle housing at about the 10 O,clock and 2 O,clock position. Stamped with a die stamp.
KA.

Thanks! I'll look for it.

My set up will be a 1275, Datsun 210 5 speed and hopefully a 3.9.

 

[Gundy]

Well, it took about 3 hours in 97 degree temps but the swap is a done deal.
I like the 4.2 a lot better. Now I have to watch out for accidental wheelspin. Pulls the driveway with no clutch feathering.
5200 rpm at 65mph.
Speedo is still a tad slow according to GPS but I can live with 3 mph slow indication at 60.
I know my 0-60 is significantly improved.
Both diffs had numbers stamped on the top of the hogshead.
If needs change it's good to know the 3.7 is stored...just in case.

 

[Hap Waldrop]

The 9/38 is the fraction for a 4.22.

The roads we will driving these cars, on, you don't need 4th gear for anywhere, add to the fact, David has a racing straight cut close ratio gear set, which has a taller 1st gear, theres a couple of corners on 276 towrds Brevard, going towrds brevard, you have to really get down as low as you can go to carry good speed, for me in the MGB it second gear with my foot planted.

 

[Gundy]

Moved to another thread. (see: my friend stinks)

 

[nomad]

Rechecked my diff's to refresh my memory. All are stamped in the 10 O,clock position. Some are stamped on th case right in front of the diff to axle bolt hole and some are stamped on the outer machined rim that contact's the axle housing. All are at 10 O,clock. All are 3.9 or 4.22 so I can't say about the 3.7. I agree with the comments above on over gearing and have stuck with the 4.22 with the 5 speed and a near stock 1275. Steep hills call for a down shift and 75 on the interstate is just nicely up in the powerband in 5'th.
Have fun scrapeing grease!!
Kurt.

 

[John Moore]

Thanks guys! I'll go scrape grease on mine and see what I find.

 

[losmorob]

Does anyone have a gear chart showing 210 vs ribcage ratios? I recently installed a 3.7 and i also have a 210 5-speed to install. If I remember, the 210 2nd is a bit taller, but 1,3 & 4 are similer in each trans. So adding the 5 speed will just give me similer ratios that i have now + a low 5th gear for those 70+ MPH trips down flat I5 in CA.

 

[John Moore]

I found the mark on mine, it appears to be 9/38, so I would assume it's the 4:22 original... until I open it up and check the gears.

Thanks for everyones help!