Rear end
 

Hey all, the weather finally cooperated today and I got under my new truck to check what diff I have. The metal tag shows 46-13 and 3.54. Now I know that this is a GMC option 3.54 ratio. Date stamp is 63 0182, making this a 1963 built diff.
You may recall that my truck is a 1964 chevy cab and 283 motor mated to a SM-420 on a 1963 GMC chassis. The chevy factory rear ends were 3.73 standard, 4.11 and a 3.07. I need to rebuild the diff as the pinion bearing and seal are shot-and who knows what else, should I change the ratio or is this a decent combo?
Brian

Re: Rear end
 

My opinion is that your gearing is desirable as I have spent weeks trying to find a Dana differential with 3.54 gears to replace my stock 4.10...I did find a 72 chevy rear end with the 3.54 which I plan on using...anyway in my little experience it's a hard to find gear ratio - keep it.

Re: Rear end
 

Measure your rear tire from the ground to the top of the tire, you will need to know the transmission and engine also......
go here: http://6066gmcclub.com/forum/showthread.php?t=48090

posts #6 or #7

you can see how your truck performs now as it is equipped and if you changed out the transmission, engine, rear axle ratio etc.......diameter of the rear tire in inches is in the file name so if it is 24" click on the file name that has 24 in it.
speed is also shown at 1500, 2000, 3000, 4000 rpm......

Re: Rear end
 

OK, thanks.
Doing some calculations with my 28" diameter tires, I get

3.54 @ 2000rpm=47mph
3.54 @ 3000rpm=70 mph
3.54 @ 4000 rpm=94 mph

If I change to the 3.07 I get:

3.07 @ 2000 rpm=54.3 mph
3.07 @ 3000 rpm=81.4 mph
3.07 @ 4000 rpm=108.5 mph

I'm thinking I really shouldn't run the 283 at 3000 rpm to run at highway speeds of 60-70mph so I might go with the 3.07. What do you guys think?

Brian

Re: Rear end
 

:whattha: to fast

Re: Rear end
 

xxxDid you look at the torque to the ground, we have to be careful and not end up with a truck that falls on its face, use 460 ft/lbs of torque to ground as a guide, much below that and the truck will not get out of its own way....look to keeping the gears you now have and changing out the transmission....what transmission do you now have ?

If you want to keep your current stock transmission you might want to go to 3:07,
if you want to change the transmission go with a t5 .76 ratio this way you would keep the same rear end and would have enough torque to pull it and cruise 63mph at 2000rpm......

Re: Rear end
 

3:07 could work but you would be better off leaving the rear ends alone and change the transmission to a t5 with a .76 OD ratio,

t5 with 3:54 rear ends: torque to ground 486.6 ft/lbs (460 minimum)

1500 rpm=47mph

2000 rpm=63mph

3000 rpm=95mph

Your truck would be a nice highway cruiser and get better fuel mileage with the t5. Kieth

Re: Rear end
 

I'm not looking to alter from stock, I just want to find the right rear end to match the SM-420 tranny at 1:1 output in 4th, with the 283 motor. I don't want to go fast, I just want to be able to run comfortably at 70mph within the 283's "happy range". BTW, does anyone know what the economic rpm is on a stock 2bbl 283?

Re: Rear end
 

Just a quick two cents worth. 40 years ago GMC pick up trucks weren't engineered or built to run comfortably at 70mph. They were mostly designed to be work horses. And if I remember right, Chevy 283s weren't designed with lots of torque and didn't lug down well, but instead did their best work, or like you say "happy range" when the rpms were a little higher. Others here might know the exact rpm numbers, but I'd say they were most happy somewhere around 2,800 to 3,200 rpms.