Gear ratio?

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By just turning one wheel (the other on the ground) and measureing drive shaft rotations the planetaries in the differential are spinning around. With and open diff. you have to have both tires off the ground (limited slip/sure grip also) and turn the drive shaft by hand while watching that the wheels are both rotating together. If you turn the driveshaft 3 revolutions to 1 wheel revolution then its 3 to 1.

Terry
 
ram250098 said:
Great this car just keeps getting beter 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
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no you won't have to shorten the rear or anything else. it is a matter of takeing the center out of the rear and swapping in a new center section. only thing that might need to be done is the yoke might need to be changed, but that is all. it is a gear swap only....
 
the 741 case rear will hold up to "some" beating but not like the 489 would. if you are going just drive it and beat it alittle you would be fine but be warey of it. the 489 is really strong and will hold up to a 4 speed well. remember it was there for a reason....
 
One tire on the ground, one not, count by half revolutions and I'll bet you come up with a little less than three half revolutions. Check it out. less than three 2.76-2.94. three and a quarter 3.23
 
Your 741 should be fine. The most common source of failure is the axle shafts themselves ( and related hardware such as c-clips)in ANY rear ( and all 8 3/4 rears use the same axle diameter). If your concerned ( which you shouldn't be), get some stronger axles from Mancini or another supplier. U- joints fail much more often than anything else! Your pinion gear diameter is a little smaller than that used in the 742 and 489 case, but still is a larger diameter than that of a Ford 9" ! Unless you're gonna run lots of torque (500ft. lbs up) AND huge slicks, you won't have any problem.
 
Jack up 1 wheel, mark it and the driveshaft. Turn the driveshaft until the wheel has turned exactly 2 revs. The number of turns of the shaft equals the axle ratio.
 
sumbitmopar said:
Jack up 1 wheel, mark it and the driveshaft. Turn the driveshaft until the wheel has turned exactly 2 revs. The number of turns of the shaft equals the axle ratio.
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Same as I said except I count a half turn as one and only rotate tire once.
 
sumbitmopar said:
Your 741 should be fine. The most common source of failure is the axle shafts themselves ( and related hardware such as c-clips)in ANY rear ( and all 8 3/4 rears use the same axle diameter). If your concerned ( which you shouldn't be), get some stronger axles from Mancini or another supplier. U- joints fail much more often than anything else! Your pinion gear diameter is a little smaller than that used in the 742 and 489 case, but still is a larger diameter than that of a Ford 9" ! Unless you're gonna run lots of torque (500ft. lbs up) AND huge slicks, you won't have any problem.
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8 3/4s do not have c clips
 
1qwikScamp said:
8 3/4s do not have c clips
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I'm speaking of what normally fails in any rear end is not usually the pinion gear. Fords and GM rears with c-clips, or some other part such as an axle or bearing on a 741 , are more likely to cause a problem than a snapped pinion gear on a properly set up rear in a typical street car.
 
I borrowed my buddies GPS and at 100kph or 62 mph my tach is reading 4000. Now with 24" tires what am I looking at for a gear ratio? I am getting 4:61 is that right?
Thanks
AL
 
man does it wind up going down the road?? with that type of gear it ought to be winding! i am not sure if that ratio could be even had in a 741 case? with all the speculation at this point i would just open it up and look at the ring gear as it will have the ratio stamped on it.....sorry i cannot help you more.
 
ram250098 said:
I borrowed my buddies GPS and at 100kph or 62 mph my tach is reading 4000. Now with 24" tires what am I looking at for a gear ratio? I am getting 4:61 is that right?
Thanks
AL
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Yep that's right or the GPS is lying or the tach is off. The tires can't lie IF you measured them correctly. lol

See you can use the same forumla to find RPM.

GEAR X MPH X 336 / TIRE SIZE = RPM
 
sumbitmopar said:
This is for determining the final drive ratio including the transmission gearing, not the rear axle ratio.
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Hence the disclaimer of assuming a 1:1 ratio at the transmission. The spreadsheet I've uploaded has allowances for overdrive (or possibly convertor slippage using the same field).
 
I am at the point with this car where I don't want to put anymore money into it this year, three grand is more than I expected in the first place.
Problem is I am kinda in the boony's and hwy driving is a bit of a must. What is my best option to get it to be more of a all around driver?
AL
 
ram250098 said:
I am at the point with this car where I don't want to put anymore money into it this year, three grand is more than I expected in the first place.
Problem is I am kinda in the boony's and hwy driving is a bit of a must. What is my best option to get it to be more of a all around driver?
AL
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If you are going 60 MPH and the motor is in the 4000 RPM range then you definitely have a low gear. Was the car a drag racer at one point in its life? Or maybe was built to be a hardcore street bruiser?

The only way to get the hwy driving you are after is to pull the gears and switch to hwy friendly gear or fit the tallest tire possible that you can get in there... like a 28" tire. That will help some but to switch out the gears or find another carrier already setup ready to go would be the only options you have.
 
I think it was a drag car...So for those who are rearend stupid such as myself, I guess that would mean removing the driveshaft and center section and replace the center with something a bit more tame..I guess it would be easier to just bolt in a new center.
AL
 
ram250098 said:
I think it was a drag car...So for those who are rearend stupid such as myself, I guess that would mean removing the driveshaft and center section and replace the center with something a bit more tame..I guess it would be easier to just bolt in a new center.
AL
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Yes or remove the center and have someone install some tamer gears for you. That would be cheaper than dropping the car off and let someone do it and charge you labor. I personally like the 3.55 gears myself... they have a little bit of bite but not to hard on the HWY. You have a 24" tire in which I have a 28" in comparsion. Use the online calculators to find out what gear you like to run using the 24" tire. With the 24" tire and 3.55 at 70mph you will have 3500 RPM. 24" tire with 3.23 gears at 70 would be 3100RPM.
 
Figure out what you have for gears. I'm looking for some 4.30's and have 3.23's for every case 489,741,742.
 
If you're doing 4 grand at 60, figureing a little converter slippage, I'm guessing that you've got 4.55 or 56 gears. If you're doing mostly highway driving and have 14" tires then I'd highly recommend 3.23 gears. You could purchase a center section (pumpkin) with 3.23 gears allready installed and set up then drop the drive shaft, pull back the drive axles and simply change out the center section. A used center with used but good gears shouldn't cost more that 700 bucks. Ask around. A new set of gears and to pay the time to someone to set everything up on your existing center will cost nearly that much.

Terry
 
Thanks for the input guy's.
Terry do the axles still need to be removed if it is not a sure grip rear.
Mine is an open, I thought you could just swap the center section without removing the axles.
AL
 
crackedback said:
Figure out what you have for gears. I'm looking for some 4.30's and have 3.23's for every case 489,741,742.
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al here is your answer right here....crackedback is a stand up guy i have dealt with and trust. at this point you need to take your impossible to deal with gears out. i would pull the center section out of the rear and hook up with crackedback on a gear swap. 3.23's will work just fine for what you are going to be driving plus you can get them in the 489 case like you originally had in the car to begin with. win win deal there......axles will need to be removed by the way, so don't miss the oppertunity to replace all the bearings. this would be a good time as well to switch to the larger bolt pattern axels w/ larger brakes too. this would finish up everything on the rear completly.
 
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