318 cars have 8 1/4 diffs?

MBDale

MBDale

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My '79 R/T has the 318/904 combo. Will it have a 8 1/4 with 3:21 ratio? Or did the only 360 cars have the 3:21 raito? If the 318 cars came with 8 1/4 diffs then I'm going for 3:55 upgrade. Thanks!
 
MBDale said:
My '79 R/T has the 318/904 combo. Will it have a 8 1/4 with 3:21 ratio?
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http://www.hamtramck-historical.com/images/dealerships/DealershipDataBook/1979/79-Aspen_0031.jpg

Mopar_axle.jpg
 
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I've never seen a V-8 F/M/J body with a 7 1/4", that's not to say there weren't any though. On the other side of it, my dad's S6/3-speed manual '74 Gold Duster has an 8 1/4" not the expected 7 1/4", go figure.
 
Ok, I'd be just as happy with a 8 1/4 as with a 8 3/4. I've got to check this thing when I get home. Then I'll just do a ratio change.
 
You may want to get a Sure Grip in there as well if you are going in there.

If you do a ratio change you will need to change the speedo gear on the tranmission, or your speedometer and odo will be off.
 
Does a 8 1/4 rear have a tag or numbers stamped on it to tell what gear ratio you have with out taking the cover off
 
if you have 3.21 gears now, don't waste your money on 3.55's. there will be no change in performance, just higher rpm's on highway.
 
To find gear ratio, jack the rear tires up.
Place trans in neutral.
If you have a Sure Grip, spin the axle one revolution while counting how many times the driveshaft spins.
If the drive shaft spins roughly 3 and 1/4 turns, you have 3.21 gears.
If you have an open differential, only jack up one wheel.
Spin the axle two revolutions and count how many times the drive shaft turns.
 
Ahhh. Good call. I'm going to check it out, pull the cover off when I get home. 3.21 be good for the street too.Just add sure-grip.
 
Aspen500 said:
I've never seen a V-8 F/M/J body with a 7 1/4", that's not to say there weren't any though. On the other side of it, my dad's S6/3-speed manual '74 Gold Duster has an 8 1/4" not the expected 7 1/4", go figure.
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alot of 6 cyl only had the 71/4 rear no need for the beefier 81/4 and weight
 
a better way to check if you have a sure grip is to just light up the back tires ,ha ha ha you got 1 black strip or 2 ?,,with sure grips you also get some chirping going around sharp corners ive found ,,the inner wheel rotation is slower than the outside tire rotation causing chirping sound
 
MiradaMegacab said:
To find gear ratio, jack the rear tires up.
Place trans in neutral.
If you have a Sure Grip, spin the axle one revolution while counting how many times the driveshaft spins.
If the drive shaft spins roughly 3 and 1/4 turns, you have 3.21 gears.
If you have an open differential, only jack up one wheel.
Spin the axle two revolutions and count how many times the drive shaft turns.
Click to expand...

Excellent!
 
There should be a metal tag on one of the diff cover bolts with the gear ratio stamped into it.
I've torn down slant six powered F's with 8-1/4 rears and V8 powered F's with 7-1/4's.
I think all wagons and 4 speeds were 8-1/4's. At least all the ones I've had were. (4 wagons and 5 od 4 speeds)
 
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