Unknown Differential Ratio

Henry's Lady's Man

Henry's Lady's Man

Senior Member
My '30 Town Sedan seems quite a bit original, but not 100%. So, I really bought a pig-in-a-poke. From all I've learned on the Barn, and from my local club, I probably have a stock engine (though restamped...I can still read original # too, but title bears rest amp #).
My question is, I also haven't a clue as to the differential ratio. Stock is 3.78 to 1, I believe, and my speedometer reads about 7 or 10 mph UNDER what I am doing, based on traffic flow. I'm thinking the speedometer needs to be restored, and is under-reading my speed. At 55mph based on traffic, it reads 42, so it seems like a progressive mis-read. So my other thought is a mismatched rear end and speedometer. Ideas? Chuck
 
Check your odometer. Maybe with the mile markers on a country road. If your odometer is correct them you have a speedometer problem. The Auto Club of Southern California used to check speedometers with a unit that they moved from office to office on a regular schedule. I wonder if your local auto club (or maybe police department) does the same thing? Are you friendly with any police officers that could shoot your car with their radar gun?

Charlie Stephens
 
I will check it out in the morning. Thanx. That can tell me about the speedo.

Is there any kind of test to try up on the jacks, like a full turn of the engine , compared with how many turns of the rear wheels, say, in first gear? Will that give me the REAR END RATIO?
 
This was posted by QGolden;
Lots of things can change in 80 plus years. Jacking one rear wheel up and turning the engine over by hand will let you determine the ratio. Easiest to pull your plugs and get your hand crank out,
Put it in 3rd Gear.
Make a mark on the jacked up tire at 6 o'clock and crank the engine over exactly 2 revolutions. When you hit exactly two revolutions, have a friend grab hold of the tire so it cannot move, or set the hand brake or whatever.

See where the mark on tire ends up:

Between 11 and 12 o'clock, ratio is 4.11
Between 12 and 1 o'clock, ratio is 3.78
Between 1 and 2 o'clock, ratio is 3.54

These are from notes of the procedure that I took from another Barner's post. A search here will find lots of info.
Click to expand...

AS NOTED BELOW THERE SEEMS TO BE AN ERROR IN THE INITIAL MARK, MARK AT 12:00 NOT 6:00
 
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Charge your GPS at home and take it for a ride. On a straight run it will tell you exactly how accurate your speedo is.
My car had the wrong speedo gear in it.
Al Leach
 
I have one of the free Speedo apps and it works great. It has a trip computer with info like top speed, average speed, stopped time, odometer, etc.
Pete
 
Ok, Wifey just read my speed with a phone speedo app. My speedo is way off, at 57, it read 45 or so. I will look for the number on the speedo cable, and also jack up car and do the rear end ratio check like Mike V Fl suggested.

Little by little, I'm finding the "unoriginal" surprises on this A.
 
I copied this from above
Lots of things can change in 80 plus years. Jacking one rear wheel up and turning the engine over by hand will let you determine the ratio. Easiest to pull your plugs and get your hand crank out,
Put it in 3rd Gear.
Make a mark on the jacked up tire at 6 o'clock and crank the engine over exactly 2 revolutions. When you hit exactly two revolutions, have a friend grab hold of the tire so it cannot move, or set the hand brake or whatever.

See where the mark on tire ends up:

Between 11 and 12 o'clock, ratio is 4.11
Between 12 and 1 o'clock, ratio is 3.78
Between 1 and 2 o'clock, ratio is 3.54

These are from notes of the procedure that I took from another Barner's post. A search here will find lots of info.
Click to expand...



Is it accurate? I must be doing something wrong..

Marked the tire at 6 O'clock

Transmission in third gear.

Two revolutions with the hand crank

Mark on the tire is at 5 O'clock after two revolutions of the engine and one plus revolutions of the tire......

I did this three times...same result each time...

???
 
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So I found this on another site...

1) Jack up one rear wheel and chalk-mark the 12 o'clock position on the tire. (Make sure the slack is out of the drive train)
2) Hand crank engine 2 complete turns.
3) See where chalk mark on tire ends up.
A) Between 11 and 12 o'clock, ratio is 4.11
B) Between 12 and 1 o'clock, ratio is 3.78
C) Between 1 and 2 o'clock, ratio is 3.54

This makes sense...

Notice the mark is made at 12 O'clock instead of 6 O'clock

In that case my mark would have ended up at 11 O'clock after two revolutions of the engine..

That would give me a 4:11 ratio which makes sense for a ccpu...

EDIT...typo on the ratios...it's correct now...
 
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1955cj5 said:
So I found this on another site...

1) Jack up one rear wheel and chalk-mark the 12 o'clock position on the tire. (Make sure the slack is out of the drive train)
2) Hand crank engine 2 complete turns.
3) See where chalk mark on tire ends up.
A) Between 11 and 12 o'clock, ratio is 4.11
B) Between 12 and 1 o'clock, ratio is 3.78
C) Between 1 and 2 o'clock, ratio is 4.54

This makes sense...

Notice the mark is made at 12 O'clock instead of 6 O'clock

In that case my mark would have ended up at 11 O'clock after two revolutions of the engine..

That would give me a 4:11 ratio which makes sense for a ccpu...
Click to expand...
Thanx for the correction, 1955cj5. That will save time and confusion! Chuck
 
Your cable is working. On the torque tube where the speedo cable attaches is a "turtle" housing the gear that drives the speedo cable. Remove it and count the gear teeth. It may be the wrong gear giving you the false reading. Replacement gears are available. Also try doing a search or someone may chime in what the correct gear (# of teeth) is.
Paul in CT
 
I'm guessing your speedo magnet is weak and needs to be adjusted to get a correct reading. IF your odometer is correct. Odometer is mechanical driven and the speedo is residual magnetism .
 
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