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What ratio rear end?

GTXMopar

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I have a stock 1969 Road Runner trying to find out what ratio rear end it has?The numbers I see on it are 2070742 12 and looks like 6435 or 6y25. Does these numbers mean anything?I don't have anything on fender tag,car was built in St. Louis.
 
First of all who knows what could be in there after 50 plus years. Two choices. turn the pinion and count how far it goes for one tire revolution. Or depending on the diff pull the cover or drop the center section and count the teeth.
 
Make sure both tires turn a revolution when you count the pinion rotation. If it has a suregrip both tires should turn the same direction "if" it is functioning. Standard RR equipment was 3.23 optional 3.55 or 3.90. If the gears have been changed it could be 2.76 or 2.94 or aftermarket gears ?
 
742 makes it the large pinion 8 3/4, and will likely have the good clutch sure grip, if it has one.
Three stock gear ratios for 69 runner, 3.23 open or sure grip, 3.55 or 3.91, both automatically a sure grip.
With both rear tires off the ground, manually turn one tire. If other tire turns same direction, 95% chance it's sg. If it turns opposite direction, it's open, and 95% chance it's 3.23.
Assuming it's a sure grip, with both tires off the ground, in neutral,
turn the rear tires one turn, and count driveshaft revolutions. 3 1/4 will be 3.23, 3 1/2 will be 3.55, almost 4 will be 3.91.
If the gears have been changed, all bets are off.
The numbers you mentioned have nothing to do with gear ratio.
(Too bad it's not a lynch road car. Rearend ratio code is on the fender tag.)
 
Did you check each bolt on the centre section to see if the little metal tag is there?
 
Put floor jack under the housing, the rotate a tire, if both tires turn in the same direction, it is a limited slip. Then rotate the tire one revolution while counting the number of revolutions the pinion makes. That tells you the gear ratio, no matter where the car was built.
 
With open rear end, you need to keep one tire on the ground, or have assistant hold one tire to keep it from rotating while turning the other tire to count drive shaft revolutions.
 
If keeping one tire on the ground so it can not turn backwards with a open rearend you will need to turn the other tire two full revolutions and count the pinion...or do one tire rev and take pinion count times 2.
 
I have a stock 1969 Road Runner trying to find out what ratio rear end it has?The numbers I see on it are 2070742 12 and looks like 6435 or 6y25. Does these numbers mean anything?I don't have anything on fender tag,car was built in St. Louis.
Put it on a hoist or axle stands with transmission in neutral. Put a chalk mark at the pinion end of the driveshaft (parallei to the driveshaft), and another chalk mark on the inside of one of the rear tires. Turn the tire, counting how many times it goes around for the driveshaft to complete one revolution. 3 1/2 times would be a 3.55 ratio, 3 1/4 times would be a 3.23 ratio, and so on.
if both rear wheels turn the same direction, it is a posi rear end.
 
It's now been 5 days. Are you going to keep us in suspense forever?


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742 is the 1-3/4" pinion; the "large" pinion would be the 1-7/8" in the 489 case, yes?

As noted above you need to either rotate the wheels and count driveshaft rotations; or open up and drop the center section and count teeth; to determine what ratio you have (unless one of the center section bolts has a metal tag still on it...but even that could be iffy, someone could change ratios but reinstall the tag to keep the look "correct").
 
I call em this:
741-small pinion
742- large pinion
489-tapered pinion.
To me, it's a toss-up between the 742 and 489. 742 usually has the better sure grip.
741, imo, is only marginally weaker than the other two, I'll buy and use any 741 i can get cheap.
 
I have a factory 489 in my wagon, 3.23 SG. Never failed me yet, and that was even including a decade or so hooked to a crate 360/380 and a reverse-manual 727, running low 13s on street tires (and the wagon is a heavy car - full interior, all glass, stereo, lights, wipers, you name it, it's there and working)! Never busted anything other than a tapered outer bearing...which I replaced with sealed bearings on both sides and never had another issue with it.
 
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