1972 rear ends

[1963chev]

Which rear end ratio will get better gas mileage and be the best overall choice for a non hot rodder, non towing, daily driver.... the 373 or the 307.6? I know all you slammers don't care about highway mileage but be good to an old man and help him out cause I've been getting too many conflicting reports. Any other positives or negatives on the two rear end choices hopefully going into my very slightly lowered 63 would be helpful. I want to cruise at 120 kmh (80 mph)!

[Chevyboy1962]

I'm not sure on the gear ratios, but the 1972 rearends are 2 inches longer than on the 1960-66 trucks (1 inch longer at each end).
David

[1963chev]

I have been told the 72 into the 63 is an easy swap....... just needing to cut the 72 leaf mounts off and weld on the 63 coil mounts. Maybe some change to the shock mounts and pan hard bar but no one has said they are different lengths. I have both the 63 rear end and the 72 so will measure them tomorrow to be sure. I hope you are mistaken. Thanks.

[Rogue]

Yep, 2 inches wider.
Ill keep my 3.73 Posi, and either buy the pricey new axles or have mine redrilled for 5 on 5.

[zenish]

71 and 72 is 1.5" wider.i think it will bolt in if you get an adjustable track bar.if your truck is a v8,then a 3.08 gear is good.

[fchava]

if you really want to cruise at 80 then you need the 308's, or an overdrive trans.

[1963chev]

Well I measured and the 1972 3.08 really is 1.5 inches longer than the old rear end that was in the 63. Is that going to cause me problems?? Will the tires hit the fenders? What other problems are in store? Maybe this 72 onto the 63 frame and body is not the best idea.

[ed455]

I have the 72 rear in my 66 and it will work. I think it improves the look/stance as the wheel wells have a more filled in appearance. You need to pay attention to your wheel back-spacing though. I had to roll the inner lip of my stepside fenders a little.

[bpmcgee]

To answer your gear ratio question -- obviously the 3.08s will give better mileage. I made the switch from 3.73s to 3.08s and I don't find a problem.

Note that I have an SM465 tranny, though. Lowering the rear end ratio actually helps make the granny first a little longer. People with TH350s, with it's much higher first gear ratio, might have a much different opinion on the swap.

Brian

[lngbed71]

Why not change just the ring/pinion and not worry about width at all?

[ed455]

The other benefits to the 72 rear end is 5 lug axles and MUCH bigger brakes.

[carryallnut]

Originally Posted by lngbed71

Why not change just the ring/pinion and not worry about width at all?

That's what I did, put a 3:07 in and took the 3:73 out. 292 with a 4-speed and I really like it. You cant just to a ring/pinion you have a utilize the entire carrier , it's different from 3:73 and up (with 4:11 you can use the old carrier).

[Slammed66]

it would be easiest to get a 3:07 ( good cruiser/highway gear) coil spring rear end, bolts right in , I have a 72 rear end under my 64 no issues what so ever

[dubie]

My 72 runs a 3:73 with a 350 and Saginaw 3 speed. I would prefer to have the 3:07 rear only because the 3:73 causes some higher RPMS on the highway, but you can sure burn some tires with the lower gears

[1963chev]

Yea bigger brakes on the '72 rear end are going to be a plus. But ED455, What do you mean "pay attention to the wheel back spacing? (I have a '63 short box, Fleet side.)

Hey bmpcgee Brian do you think there is a disadvantage with the 3.08 matched to a 350 turbo trans? That is the way it is in the stock 72 that I am getting everything off of.

The 3.73 with an overdrive trans would probably be the best of both worlds.

[ed455]

I was referring more to the width of the wheel. An 8 inch wheel with 4 inches of backspacing sticks out 4 inches beyond the end of the axle. The same width wheel with 3 inches of backspacing sticks out 5 inches and so on. Too little back spacing on a lowered truck and you'll have fender rubbing issues. This is especially true with the 72 rear end in a 60-66 series truck which originally had a rear end that was more narrow by 1-1/2".

I have 8 inch wheels with 4" back-spacing and the truck is lowered 4 inches in the back. At rest, there was to fender rub, but when I launch, the fenders would have come down on the sidewalls a bit, cutting them. I simply rolled the fender lips in a little (stepside).