275 45 18 on rear - speedo question?

[efi4eye]

So I've been searching for compatible tires and got a great deal on some 255/45/18s for the front and 275/45/18s for the rear. I have a 300A so no traction control. My question is - will I have any issues with speedo accuracy having a 27.7" rear tire vs. a 28.5" OE tire?

If so, how can I correct for this? Can it be corrected in the tune, or?

[SC Cheesehead]

You'll need to adjust the revs per mile via your tuner to correct speedo variance.

[efi4eye]

You'll need to adjust the revs per mile via your tuner to correct speedo variance.

If I have a locked tune, can I still change this independently?

[efi4eye]

anyone? Bueller?

[Krytin]

Not if it's locked. Go back to tuner and ask for a revision for tire size.
I've got 255/45/18's front and rear for the last 6 years - with the rev per mile set for the 27" tires the speedo matches the GPS & police radar dead on!