Speedo correction

on my 86 firebird I bought it with huge tires in the back, and th350 trans swapped in aleready.

problem is my speed reads double what im going, the Rear tires are 275/60/R15 could this taller tire be part of my speedo problem? Im buying stock rims soon with right sized tires.

is there a gear or a dif cable I can switch in the trans to correct the speedo?

My guess would be your rear axle gears have been changed or the transmission expects a different axle ratio and tire combination. The speedometer gears must not be right for the tire size and axle ratio you have.

I drive a 1982 Trans AM 350/350 and my winter tires are 275-60-15. When im going 45 MPH the speedo reads 40 MPH. To figure out what drive/driven gears you need you will have to know what the axle ratio is, and what tire size will be used.

I know my rear gear ratio is the stock 3.42, what drive gears and tires do I use with this?

I know my rear gear ratio is the stock 3.42, what drive gears and tires do I use with this?
If it is just a plain jane firebird then the tires would be something like P225 70R 15...or there abouts.