Adjusting for tire size using SCT tuner

[Wish I Had a Nice Car]

About to put new tires on the car, but im not going with the crappy 235/50/18's. Now i know you can adjust the tire size calibration for the car using the SCT based on Revs per mile. I am going with a 255/45/18 on the front and a 275/40/18 on the rear.

The stock Revs/mile is 763.3. The new front will be 769.5 and the new rear would be 780.1.

The question i have is how would i go about entering this information into the Tuner for tire size? Would i just set it at 780 because thats the diameter of the rear tire? Or would i need to go in between the 2 at 774.5?

I am using http://www.1010tires.com/TireSizeCalculator.asp?action=submit for the Revs/perMile information

 

[Vapour Trails]

Diameter of the rear tire (780-785 is about right).

 

[BAKnBLK2010]

I wouldn't do anything. All three size tires you mentioned are right at 27 inches tall. Just put the new tires on and enjoy.

 

[JDM74]

The anti-lock brake system is based on the tires being the same circumfrance. Any difference in tire diameter front to rear above a certain point is going to set an ABS light. you should be fine with these though if they are both 27" tall.