determine diff. ratio,closed drive shaft

is it an open rear end? if so jack up one wheel and rotate it two times while some one counts how many times the drive line turns
 
Thank you for responses.
John, I will give your info a try.

Further : the car is complete and running. Drive line is original I think.

I will post my findings
 
In the title you say enclosed driveline.

If the engine and box are in the car and you have a hand crank you can jack up one wheel and with the car in top gear count how many turns of the engine it takes to make the jacked up wheel turn exactly 2 revs.

If not there is another way, but you aren't exactly overburdening us with information.

Mart.
 
What turtle is installed? Drive the car and check the speed by timed mile markers, gps, etc. If the speedo is acurate, you have the right turtle. 21 would be a 4.11, a 19 would be a 3.78 and a 18 would be a 3.54. Not for sure as wheel dia can throw it off but it is a start and you can have a ride.
 
Ratio should be stamped on the lower reinforcing rib of the center section. If not, remove both plugs from the housing, drain gear lube, use a rag in a screwdriver and wipe each ring gear tooth (with car jacked up and turning a wheel). Then mark ea gear with chaulk counting teeth as you go. 34=3/78, 37=4/11, 40=4/44, 41=4/56 and I do not remember what the 3/54 is as most of them cam in Mercs.
 
I have not found stamped numbers on diff.
Counted teeth on 'turtle' 19. speedo checked against GPS - seems accurate, conclusion differential is 3.78
Real problem is when going 30 mph my tach reads 2500rpm. have no reason to doubt it because it was my ear that made me investigate.
At 2500 rpm car should be going at least 50 mph.
I had not driven car before having transmission rebuilt by a hobbyist. He replaced at least 1 gear. Is it it possible to effect a trns output so that 3rd is less then1 to1?
This is a 34 Ford V8

Thanks, Keith
 
I use a different way---put mark on crankshaft pulley, mark tire (front is ok if same size as rear)--or make note of where valve stem is (I start with it at the top by feel), put in direct gear (3rd for original trans), then I push car---rotate tire while watching crankshaft mark ---when it makes 1 revolution and you have counted 3- and whatever --3-1/2 is about 3:54 ---3-3/4 = about 3:78 ---a little more than 4= 4:11
 
welcome to the 'Barn...hope you get it figured....sounds like way too high rpm for 30 mph.....you in 3rd gear??...lol....and do you have pics of your car??....we love pics....Mike
 
retep said:
I have not found stamped numbers on diff.
Counted teeth on 'turtle' 19. speedo checked against GPS - seems accurate, conclusion differential is 3.78
Real problem is when going 30 mph my tach reads 2500rpm. have no reason to doubt it because it was my ear that made me investigate.
At 2500 rpm car should be going at least 50 mph.
I had not driven car before having transmission rebuilt by a hobbyist. He replaced at least 1 gear. Is it it possible to effect a trns output so that 3rd is less then1 to1?
This is a 34 Ford V8

Thanks, Keith
Click to expand...

With a 3.78 axle and 28 inch tall tires you should be going about 55 mph at 2500 rpm
 
Is it an electic tach? It is probably set up wrong or wrong application.
Most conventional older 3 and 4 speed boxes are 1:1 in top. as has been pointed out. The various Ford 3 and 4 speeds up to the early fiftys are all direct in top, as far as I know.
FYI (many modern 5 speed boxes (Jap) are 1:1 in 4th and od in 5th)

Mart had it covered on how to do it.
1 turn of your engine equals 1 turn of the pinion. If 1 wheel is on the ground (and still) then it takes 2 turns of the opposite (jacked up side) wheel, to equal 1 turn of the crown wheel.
So you are looking for 3.5 to 4.4 turns of your crank handle to get a mark on your tyre back to the same place (second time around)
Hope that make sense.
 
You have not mentioed what car you have, year and tire size. For instance early to mid year 32s do not have numbers stamped on the rear end housing. Tire size is a factor in rpm. The only absolute way to determine ration is to count the ring gear teeth.
 
Ford Barn.jpg

I will use Martins method. I suspect based on my experience the ratio will come out in the 6 range.
Tire size 27.68
I am assuming transmission and drive line are 34. Engine is from 36 Ford.
I will update with my findings.

Keith
 
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You might find your rev counter is set to 6 cylinder. I don't think Ford made a ratio available that is a 6-something. It probably has 4:11's like my 33.

Mart.
 
very nice looking car.....if you could see serial number on transmission bellhousing, above inspection cover, then you could tell what year trans....any more pics??...Mike
 
So as a recap I have counted teeth on speedo at 19 should mean ratio3.78
Found number stamped on diff is 55 means nothing to me. Hope its not the number of teeth on the ring gear .

Am going out now to do some car raising and counting wheel turns.

Back soon

Keith
 
YUP ! it's probably the wrong speedometer gear tooth count. I don't remember the correct tooth number for he 4:11.
 
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