Speedometer gear question??

I changed my Tire size from stock size 225/70r15 to 255/60r15 how do you calculate this to get the right colored gear for the speedometer??

Actually the cahnge showing on your speedo is neglible. it should only read about 1 MPH faster at 60 MPH

At 55 mph my speedo reads 65 mph so maybe the previous owner changed the gear i know stock is 3:08, I just figured it was the tires?

Check your email

Thanks for all the great info, I also will be looking to get that 140 mph speedometer thru Red line.