Gear ratio?

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ram250098

ram250098

69 Barracuda
Joined
Nov 25, 2007
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Location
Cookstown Ontario Canada
I had the rear up today and marked the driveshaft, I spun the back wheels and the shaft turned 1 1/4 turn. Would that be a 3:23 ratio?
I can't remember how to figure it out.
AL
 
Something doesn't sound right there.


Try turning the drive shaft by hand, 1 full turn and count how many time the tire goes around.

This will give you a ballpark guess.
 
Something doesn't sound right there<snip>
I know it seemed very odd to me as well, I remeber spinning the wheel and counting the revolutions of the shaft.
AL
 
I think that 323 means that one turn of the wheel while holding the other still the shaft should turn 3.23 times. I think
 
Well if you turned the drive shaft once and the tire went around once that would be 1 to 1.

You are saying the drive shaft turned 1.25 and the tire went around once that would be less that 1 to 1 unless Im confused, and that happens sometimes.
 
i believe bumnround is right

turn wheel and count driveshaft turns

1 wheel turn should give 3.23 drive shaft turns?
 
I jacked up the drivers side and spun the wheel and got 1 1/2 turns of the shaft. I jacked the passenger side and got 2 revolutions of the shaft for one wheel spin..still seems odd though..any thoughts.
AL
 
Some thing is not right. Did you try having some one hold one wheel from turning as you turn the other one? That may help. It is ether that, you are not doing it right or some thing is broke.
 
If it is not a sure grip, you have to stop one wheel from turning and count the revolutions of the shaft it takes to make the free wheel spin one complete turn.
 
I had one wheel on the ground so it would not spin, best I could get was two revs of the driveshaft for one wheel spin. I can't get under the car enough to spin the shaft but that should give me the same result. The car runs and drives fine with no rear end noise at all.
AL
 
Gear Ratio = (RPM x Tire Diameter) / (MPH x 336)

Note: This is assuming a 1:1 final drive in the transmission
 
Thanks for the help guy's. I really think this is strange.
The tranny is a 4 speed with a 8 3/4 rear 741 case the tires are 14", 235/60R/14s
If the speedo is correct at 80-100mph I am reving around 3 grand.
AL
 
ram250098 said:
Thanks for the help guy's. I really think this is strange.
The tranny is a 4 speed with a 8 3/4 rear 741 case the tires are 14".
If the speedo is correct at 80-100mph I am reving around 3 grand.
AL
Click to expand...
The critical tire dimension for these calculations will be the diameter of the tire itself (from ground to top of rubber). It'll probably be somewhere between 24 and 27 inches. I've got a spreadsheet for this stuff somewhere. Maybe I can upload it for easy future reference.

Edit to add: I've added a calculation for Gear Ratio given the other variables. Let me know if it doesn't work properly. It seems to make sense, but my brain has been acting up again.

View attachment crap.zip
 
Yes right right it is 24" to top of tire. The spredsheet would be great. This is really bugging me not knowing. The rpms seem high and I am wondering is the gears were changed and if so why not go with a sure grip, the frame has been tied and she has a few goddies in her but first thing I would have done would be the rearend. Big auto swap meet this weekend but from past times very little Mopar stuff.
AL
 
Gear Ratio = (RPM x Tire Diameter) / (MPH x 336)
So 3000 rpm and 24" diameter 80 mph x 336
So roughly if I get ...gear ratio- {81000} / {26880}
How do you reduce it down?
Thanks for the math lesson...I don't think I am ready for Smarter Than A Fifth Grader
AL
 
ram250098 said:
Gear Ratio = (RPM x Tire Diameter) / (MPH x 336)
So 3000 rpm and 24" diameter 80 mph x 336
So roughly if I get ...gear ratio- {81000} / {26880}
How do you reduce it down?
Thanks for the math lesson...I don't think I am ready for Smarter Than A Fifth Grader
AL
Click to expand...
I'm getting 3.01

I've attached the spreadsheet as a zip file in the post above. It shows up as a link. I'm into mathematical shortcuts when I can make/find them.
 
al your car should have had a 489 rear with either a 3.23 or 3.55 ratio. these were the only 2 available for the 340 cars. if it is a 741 then it has been swapped out.
 
my bet it is a 2.94 from a c body, or b body wagon. a 2.76 is a little more common and was easily available in alot of cars as well.
 
Great this car just keeps getting better and better with all these surprises LOL. So the rear would have to have been shortend? This seems like a lot more work, why not just swap another A body rear?
I guess I have to start looking for a better rear.
Thanks so much for everything guy's, this site is the best.
AL
 
Correct me if I'm wrong, but I thought that with a non sure grip rear, you leave one tire on the ground, and rotate the other tire 2 complete revolutions. The drive shaft revolutions would then indicate rear end ratio.

Make sure to mark one of the u joint caps so you have an accurate count of revolutions. It is very easy to loose track of the turns of the driveshaft.
 
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