Your right the overdrive changes, if you cut the tires as you would for the 24t and then put the 25t on the car will be under driven say 1/2 percent, say you cut your tire at 59mm frt and 61mm rear. But some of the drivers like the tire diameter are closer together so the rear doesn’t wear faster than the front and tires will be more equal from the start of the race. To acieve the same over drive the tire dia differance is around .5mm differance from frt to rear with the 25t. It is not the most correct answer, but it easy to explain it that way some racers try it.., lol. I tried it and I personally didn’t like it.