With a 1.5% difference in free speed, assuming it is not an issue of Victor calibration (much lighter of a fix than a second gear
), at 12.75:1, one side of your drive will spin freely at about 417 RPM, and a theoretically slower CIM will spin at 410 RPM. This assumes you have only 2 CIMs in your drivetrain; I'd gander that 2 more CIMs would "average out" this effect to make it even smaller.
With 6" direct driven wheels, and an assumed 81% efficiency, that's about 8.84 feet per second on one side and 8.69 feet per second on the other. That's a difference of about 2 inches per second. Turning scrub would probably stop your drive from tracking two inches in one direction over 9 feet.