A friend of mine is dropping an LT1 into his '67 convertible. We have to get the computer reprogrammed for the rear gear ratio and tire size. Last night I put the rear end in the air so I could figure the gear ratio. I laid under the car and spun the driveshaft while I watched the wheels turn. What's odd is that for every 2 turns of the driveshaft the tire turned 1.25 times. This turns out to be a 1.6:1 ratio. I tested it several times and came up with the same results every time. Did a first gen come with a ratio this shallow?

The car was originally a smallblock/TH350 combo.

-Chris

 

Are you sure you're doing everything right???

Are BOTH tires turning evenly?? If it's an open end rear end (non-posi) it may be a little more difficult, because when you turn the driveshaft, only one of the wheels might turn (or one can turn slower than the other). Then you may actually have to turn the tires (together) and count the driveshaft turns. Keep in mind, it's easy to screw up counting the driveshaft turns, because usually you gauge by the U-joint, and it's easy to count 1/2 turn as 1 full turn, since it will look in the same position (ie: horizontal or vertical). Have a buddy tell you when you've turned the tires 1 full turn, and determine how many turns of the driveshaft it took.

I've heard of high ratios like 2.11 or something (foreign cars I think?? ) but I don't think GM ever put anything higher than 2.56's in Camaros. DEFINITELY NOT a 1.6?!?!

IMO, you mis-counted................try again!

 

I'm thinking it's a 3.23. When you have an open diff, and only one tire is turning, the gears in the carrier will divide by two. I learned this when I was looking at a diff from a 56 chevy w/3 speed overdrive. Figured if car had OD, the diff must have 3.7 or 4.1 gears. Lifted one tire and turned. Got 2 turns. Bought it anyway, disassembled, counted teeth, came up with 4.11.

 

I saw a Belinetta once with 2:43 gears, and it wouldn't spin a tire in the dry pavement. lol, The gears were going out behind a 305, and it wasn't a real performer to say the least.

 

Well, I've done this a couple of times before, so I'm pretty sure I did everything the same as before. Both tires were spinning, and they were going in opposite directions, so it has posi (right?), and I was counting the turns off the yoke, but I made sure I was counting right. And even if I was only counting half a turn, wouldn't that mean that the gears were twice as shallow instead of twice as deep?

I did try to turn the driver's tire and count the turns of the driveshaft, but the tire would spin a little (maybe 1/4 turn) before the driveshaft would begin to turn, so I scrapped that idea quick.

Like I said, I've done this before with good results, but I'm stumped on this one. Could an old worn posi have anything to do with it? Any other suggestions other than popping the diff cover and counting the teeth?

Thanks for the help!

-Chris

 

I just re-read this and you got me thinking. If the car had an open diff would the tires turn in opposite directions? If so then it's possible it's an open rear. I did watch to see that both tires were turning, but I didn't watch to see if the were tuning the same amount.

Thanks again!

-Chris

 

Aahh!!!!!! There's the problem!!!

Sorry dude, but it ain't a posi. With a posi, BOTH wheels turn the SAME direction at the SAME time. This is because the posi unit essentially "locks" the two axles together (through friction) and thus when you accelerate, both wheels apply the power to the ground evenly (or at least without one wheel slipping and slowing you down). Since you're dealing with an "open" diff, each wheel will always (usually ) turn the opposite direction of the other wheel when they're up in the air and you're turning one of them.

SO, what you've gotta do is mark BOTH tires (maybe using a piece of duct tape or something ) and turn them together, evenly, keeping your mark lined up on either side, and count the # of turns of the yoke after making ONE full turn of BOTH tires together. Get what I mean?

 

Yup. Thanks, dude! It's been a few years since I've had to do this.

-Chris