Rear End

[skysoldier173rd]

Anyone know what the factory gear ratio would be on a 60 GMC Short Bed Stepside with a four speed with granny gear would be. It had the old 305 v6 engine. Doing a 350 with 700R4 and wondering what i will end up with as far as cruisability and on the highway.


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[72bigred]

Not sure what all was avail.. Mine were 3.90

With the 700.. not to shabby, I personally swopped out for a later 12 bolt.. just stronger, easier to find parts for and of course a range of gear ratio's.

[Captainfab]

IIRC the '60-'62 GMC's had the 55T rear end, which most of those came with 3.90 gears.

[pmercer]

I have a '61 GMC (Canadian) and it too has 390 gears. It had to find parts for, not the same as the older chevy cars. The front of mine comes out as well as there is a rear cover. Each has 10 bolts holding it. I could not find replacement gears for it anywhere.

Peter

[raycow]

A GMC would have a Dana 44, which has much better parts availability than the old Chevy rear. Most of these were not geared as low as the Chevys. I have seen them with 3.07 and 3.54.

There is no need at all to guess about the ratio.

Put matching marks on the pinion yoke and differential housing. Jack one rear wheel off the ground, put the transmission in neutral, and the parking brake off. Turn the wheel exactly TWO revolutions and count how many times the driveshaft turns, estimating any fractional turn as closely as possible. That number is your axle ratio.

If the wheel is difficult or impossible to turn, you may have posi. In that case, jack both rear wheels off the ground and turn either of them only ONE revolution. Count the driveshaft turns the same way as for an open rear.

Ray

[hoog]

I'll bite....why two revolutions?

[raycow]

Originally Posted by hoog

I'll bite....why two revolutions?

Because of the differential. If you want to look at it another way, you could turn BOTH wheels ONE revolution, but this means you need two people. It is just easier leave one wheel on the ground and turn the other two revolutions.

Now a working posi won't let you do that, so you have to get both wheels off the ground. When you turn one wheel on a posi, the other wheel turns with it (unless the posi clutches are destroyed).

Ray