Speedometer Ratio adapter

[AASmedic]

I have a discrepancy between my speedometer and the actual speed because I changed out to bigger tires. I know that I can install a speedo gear ratio adapter to correct the discrepancy. It is off about 10% (Reads 60 when Im only doing 55). The question is finding out the correct ratio. I have a kit that I purchased recently that I can use to build the needed adapter and it came with many different gears but unfortunately the instructions are vague or non-existent. Any help would be greatly appreciated.

thanks,
kevin