[fiveohpatrol]

the taller the tire, the lower (numerically) the gear ratio. Think about it, it's all about getting your engine RPMs up. If you had a small tire before, then went and put a taller one on, the taller one will be spinning slower than a small tire at the speed. A tire with half the circumference than another will need to spin half as much to cover the same distance.

A general rule of thumb is to subtract 0.1 from your numerical rear gear ratio for every 1" increase in tire diameter. So if you have 3.73's with a 23.5" tire, thats roughly equivalent to having a 3.48 gear with your 26's.

The real question is, what the heck are you doing with 23.5" tires on the rear of a mustang?
If you're concerned about it, just put in some 4.10's or 4.30's