rear differential question

pbarge

Junior Member
Location
Texas
I have a 1960 truck that has a rear end from a 1970 chevy truck. It is a 12 bolt, 6 lug, 3:07:1 ratio. All was fine until the TB 350 tranny went out and I opted for a 700r4 as a replacement. I now have too high a ratio and feel that I need about a 3:43:1 rear end to make this right. So the question is, is what year(s) model chevy trucks would have this type of rear-end ratio that the gears could be pulled and installed in this 1970 rear end without major modification?

Thanks!
 

Alan

PlowSite.com Addict
Don't rule out going to 3.73 gears behind that 700R4. 3.4? will be ok if you're not towing or plowing, but if you load very heavy they will still be a little too tall.

I'm not positive, but I think you will be better off swapping the whole rear rather than setting up the gears in the housing. I would think that a 12 bolt shold be fairly easy to come by, or even a 10 bolt. Granted, the 10 is supposed to be a weaker axle all around, but keep in mind that GM ran 10 bolts in the front of K20s and they stood up to a lot of abuse.
 
OP
P

pbarge

Junior Member
Location
Texas
rear end

Alan,

Thanks for your reply. When I installed this 1970 rear end into my 1960 truck i had to cut the perches and re-weld them to fit. I would rather not do that this time and just change out the gears. 3:73 would be an option, but I tow very little and want the best compromise between performance and gas mileage. Would you happen to know the years that the gears would swap out?

Pat
 

MTCK

Senior Member
Location
Fairbanks AK
Depending on how tall your tires are, and how much cruising you will be doing, a lower (numerically higher) ratio than 3.73 could even be ok. With the turbo 350 you had a final drive ratio of 1:1 coming out of the transmission. With the 700R4 in overdrive, you are 1:.7. So to turn the same RPM at the same speed with your new transmission, 3.08/0.7 is 4.4. Soooooooooo you could go with a 4.10 gear ratio, turn less RPM than you were before, and have lots of get up and go. 3.73 would be fine also, I'm sure, and you'd see more MPG gains. To swap the gears, if you don't do it yourself, you're probably looking at about $500 parts and labor.

Marcus
 

75

PlowSite.com Addict
I agree with Alan that swapping out the rear axle as a unit is likely the better way to go. Changing the gears themselves is a bit more involved than simply yanking one set out and stuffing another set in - they need to be set up with the correct clearance and backlash in the gears, otherwise they won't last very long.

Since I'm not a mechanic but work in the metal fabricating field, naturally I would prefer to relocate spring perches over fiddling with gears and shims. And, if you can locate a couple of different ratio rear axles (say 3.73 and 3.4?) fab'em up and try one, if you don't like the results try the other one.
 

staley52

Member
Location
Iowa
why is this rear gear wrong now?

the 700 has a lower 1st gear than the 350 almost like a granny low for an auto. if you are worried about it shifting with a load dont use OD D is still 1 to 1 and I have seen trucks with 2.83 rears and a 700 get around fine if its not broke dont fix it

also I would rec getting a lock up delay kit or a non locking convertor for it if you plan to work it
 
Top