determine diff. ratio,closed drive shaft

The speedo gear tooth count also depends on tire size. My '35 uses a #19 turtle with 3.78 gears and 600 x 16" tires. A '33/'34 with 17" tires may use a different turtle gear # for that same ratio. I believe there is a chart in the Green bible that gives turtle gear # based on year of car, tire size, and rear gear ratio.
 
Back.
I jacked up car on one wheel, used it to turn engine over twice (marked crank), Tire turn required less then one (between 11 and 12). My information says this should be a 4.11 ratio diff.

Is this correct?

Keith
 
Not 100% sure how to do it that way.

Jack one wheel up. Put trans in top gear. Turn the engine and count the turns until the wheel has done exactly 2 revs. The number or turns of the engine equals the diff ratio.

The ratio may be 4.11 from your calculations but because you cant say exactly how far round the wheel was it is not possible to be 100% certain. 4.11 is the most likely candidate but unless done like described above it is hard to be certain.

Mart.
 
Yes, once again agree with Mart. Do exactly two turns of the wheel, counting engine turns. The amount you turn the engine less or more than 4 turns will indicate your gear ratio directly. If you do it carefully you will be able to establish the ratio.
 
Did the 2 rev on wheel and counted crank turns at 4 + turns. Guesstimate approx 15 degrees. Diff likely 4.11.

I plan to block up rearend of car and run tach measuring each gear.
I will update post when done

Thanks to all for your input.

Keith
 
Just realised that your wheels look like 15" Merc. What size tires are you running? It may be that you could do with something a lot bigger. (for example.. I am running 7.50:16's)

Mart.
 
Kieth you may have a ? speedometer. Just thinking. The speedometer gears are set to the tire size and rear differential of automobile. Example 35 ford 600 x 16 tires will have a different speedometer gear with a #19 turtle and a 3:78 rear than a 38 with 650 x 16 tires and a 3:78 rear ,.# 19 turtle the gearing in the speedometer is different. A question is your car running 2400 rpms at 30 mph or your speedometer just telling you the engine is running 2400 rpms ?

Note : Put a electric tack on it and see what you are doing at 60 mph.
 
JM 35 Sedan said:
The speedo gear tooth count also depends on tire size. My '35 uses a #19 turtle with 3.78 gears and 600 x 16" tires. A '33/'34 with 17" tires may use a different turtle gear # for that same ratio. I believe there is a chart in the Green bible that gives turtle gear # based on year of car, tire size, and rear gear ratio.
Click to expand...

Yes the tire size does matter, but the gearing in the speedometer fine tunes the speedometer. The 34 may use a #19 turtle but the gears in the speedometer will make up for the 17" tires and a 3:78 rear or a 4:11 rear.
 
Mart. My tire size G78-15 od 27.68" Original tire size 550-17 od 28.5" your tire 31.48"
The difference in mph at 2500rpm my tire I should be going 50mph, with yours it would be 57mph. I am considering larger rear tires.
Jimvette. I am using a 1950's cam angle/tach. I need some way of checking its accuracy? As before, I am going to raise my rear and check rpms through all 3 gears at 2500rpm. I will report back.

Thanks for sticking with me,
Keith
 
I noticed the comments on tire size .... It doesn't change the ratio calculation, only how far you go per turn of the wheel.

If you can easily see the drive shaft or crank pulley turns (in high gear), I found going for 10 wheel turns (both on the ground) gives you a closer estimate of the ratio. 10 wheel turns & 40 crank turns, = 4:10 as an example. Of course you need to walk a little further :-) . A 4:11 would be 41.1 crank turns, and 10 wheel turns.
 
retep said:
Mart. My tire size G78-15 od 27.68" Original tire size 550-17 od 28.5" your tire 31.48"
The difference in mph at 2500rpm my tire I should be going 50mph, with yours it would be 57mph. I am considering larger rear tires.
Jimvette. I am using a 1950's cam angle/tach. I need some way of checking its accuracy? As before, I am going to raise my rear and check rpms through all 3 gears at 2500rpm. I will report back.

Thanks for sticking with me,
Keith
Click to expand...

Keith, you would benefit from some taller rubber. Your tyre is almost an inch smaller diameter than even the stock 17" size, but (as I'm sure you know) it is the circumference that matters, so you would be short on about 3" of travel for every wheel rev turned.

If you're stuck with the 4.11s, maybe a switch to some 16" wheels with a 7.00/6.00 combo would get 'er down the road with a bit less revvage.

You could try some L78's on the back, but really if you want to stick with 15s you would need some 8.20s.

Remember folks, big tyres = poor man's overdrive.

Mart.

PS I don't think you need to do any more checking. You know your tyre size and axle ratio.. anything else can be calculated. If you doubt your speedo accuracy you can check it with a sat nav device, borrow one if necessary, or many modern phones have a sat nav app. At 50mph your engine WILL be doing a certain number of revs despite what your tacho may be telling you.

Mart.
 
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Green bible says 22 teeth part#18-17270 with 550/16 tires and 21 teeth #B-17270-B with 600/16 and 650/15 with 4.11 gear 700/16 uses a 20 tooth gear. Dave
 
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Update; car on blocks run through gears.
1st 10mph @ 2500rpm
2nd 17mph@ 2500rpm
3rd 30mph @ 2500rpm

Also ran at 40mph, reading of 3100rpm

I will re check speedo against GPS tomorrow.

Another thing Iwant to check out is possibility of a 4.33 diff. Itwas optional for mountain areas.

Later
Keith
 
I am interested in what the 55 stamped on differential means. See attached. 34 diff.jpg

Keith
 
retep said:
Update; car on blocks run through gears.
1st 10mph @ 2500rpm
2nd 17mph@ 2500rpm
3rd 30mph @ 2500rpm

Also ran at 40mph, reading of 3100rpm

I will re check speedo against GPS tomorrow.

Another thing Iwant to check out is possibility of a 4.33 diff. Itwas optional for mountain areas.

Later
Keith
Click to expand...


Keith,

Your info is screwed up.

At a True 2500 rpm in 3rd your car will be doing 50mph with a 4.11 or 47.5mph with 4.33's.

This means there are 1 of 4 scenario's at play.

1, Your tacho is reading high.
2, Your speedo is reading low (Mine does in my A)
3, Both of the above.
4, The tacho and speedo are both correct and you have a 6.8:1 axle. UNLIKELY!

Check the speedo against a GPS or if difficult, arrange to follow a modern car at an indicated 52mph. (They're always a bit out).

That will tell you how far out your speedo is.

Once you know the true speed of the car, and knowing the gearing is 4.1:1 from the wheel turning exercise, you can then calculate your true engine revs and work out what is wrong with the tach.

We will get this one cracked.

Mart.
 
Hello Everyone, Interesting thread.

Keith, can you hop in a modern car, in idle, and rev to 2500 to get an idea of what that sounds like, then compare that to the sound of the flathead? I'm voting the tach is the culprit. Using a GPS for now, what does it sound like at 50mph? Will it even do 60? When we had 4.11:1 ratio, 60 was out of the question for the amount of strain I could tolerate, 55 was on the edge of too much for the old girl.

-VT/JeffH
 
tach.jpg

Thanks for sticking with me.

I have attached pic of said tach. It's from the 50's. There is no 6 or 12 volt selection. Internal battery is 6volt. I was pondering the idea of checking it against tach on new car. Is it possible?(electronic ignition)

I used my GPS yesterday at 30 and 40 mph and it seemed close. (maybe a mile slower(GPS that is). I'll try at higher speed tomorrow.

I guess if I want to be sure of diff I will need to count ring gears.

I shall return.

Keith
 
Okay. I have checked differential. Counted 37 teeth in ring gear, hence for sure 4.11.
Speedometer is close by GPS. out by 1 to 2 mph at 30 through 50 mph.
Haven't been able to check tach.
My conclusion is tire size.

Thanks for all your input.... I am going to let this thread die.

Keith
 
retep said:
Okay. I have checked differential. Counted 37 teeth in ring gear, hence for sure 4.11.
Speedometer is close by GPS. out by 1 to 2 mph at 30 through 50 mph.
Haven't been able to check tach.
My conclusion is tire size.



Keith
Click to expand...

I would conclude your tach is off, tire size won't get you the RPM's you seek:)
 
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