I am swapping a 700R4 tranny into a '78 Silver Anniversary L-82 C3 with the gymkhana (sp?) suspension. I think it has a 3.55 gear, but not positive. Can anyone out there confirm the ratio. I am getting a new speedo driven gear set for it. The tranny drive gear is color coded blue.

Thanks,

Dave

Hi Dave,
In the Corvette Restoration Guide there are two differential codes given. They are:
OA 2.72:1 Pos trac with standard wheels
OF 2.87:1 Pos trac with aluminum wheels
I hope this will help you

I think mine's deeper than 2.72 or 2.87. I'm turning 3500 RPM @ 75 MPH (GPS timed speed). I'll guess I'll just have to check out my tire size and do the math!!!!

Both my 78's are L82 automatics. Both 3.55 gears. My Corvette Restoration Manual shows for L82 auto only the 3.55. L48 standard is 3.08, optional 3.55. Mike

I checked out the tire size of 255/60R15 and did the math. It came out to 3.75:1 so given the GPS usually reads a little slow and plug in the torque converter slip along with a fudge factor it's a 3:55. It sure will be nice with that .7 overdrive and lock-up converter to be turning a leisurely 2450 RPM @ 75 MPH!!!

Thanks for you help. I may have more questions for a guy with 2 '78s.....

Dave

biggrin| I have a 1978 with RPO# G95 3.73:1 382 Corvette were orderd with this Op. | I run 3500 RPM at 70 MPH. My bet is you have 3.73 and you GPS is real Accurate! | |
Curtis

You could get 3.70 with a 4speed. (1979)is RPO GS4. Not sure of 78 RPO. Mike

Try looking for an ID number, bottom edge of differential carrier flange OK-3.08; OM-3.36; OH-3.55; OJ-3.70 Mike

I have an L82 auto. I couldn't see the code well enough to identify the diff but I used a gear calc and got around 3.7 (a little over acutally) Go to the link below for a gear calc. http://richmondgear.com/streetcalculators.html

When all else fails here is an easy accurate way to tell the ratio.
Lift both rear wheels in the air. Use chalk and mark the position of the drive shaft and wheels. Rotate the drive shaft and count the number of turns to make the rear wheels turn exactly one turn. The total number of drive shaft rotations is the gear ratio.

Ken Styer