318 cars have 8 1/4 diffs?

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!
 

efriedrich

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My '79 R/T has the 318/904 combo. Will it have a 8 1/4 with 3:21 ratio?

http://www.hamtramck-historical.com/images/dealerships/DealershipDataBook/1979/79-Aspen_0031.jpg

Mopar_axle.jpg
 
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Aspen500

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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.
 

MBDale

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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.
 

kkritsilas

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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.
 

Gator

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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
 

9secRR

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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.
 

MiradaMegacab

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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.
 

MBDale

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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.
 

Superpac Ninja

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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.

alot of 6 cyl only had the 71/4 rear no need for the beefier 81/4 and weight
 

Superpac Ninja

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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
 

Jack Meoff

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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.

Excellent!
 

F body Deconstructor Jim

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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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