Tires: tire size difference vs. aspect ratio difference, poor choices, rear tires

Question
I have a 2005 Ford F150.  The placard shows P255/65R17 which is what I currently have.  I am having to replace both rear tires and I have a buddy that will practically give me 2 new P235/70R17's or 2 newer P255/70R17's.  Is either one choice smart? Thanks.
Cliff

Answer
Cliff,

If you have a 4X4, then you should ALWAYS keep the same size, make, model and state of wear on your truck in order to prevent the drivetrain from binding up and eventually failing.

But if you have a 4X2, the situation is less critical.  It's always best to have 4 of the same size, make, model, and state of wear so the vehicle is balanced front to rear.  The more different the tires are, the more potential problems there are - and this is just beyond the outer limit of what would be considered "safe".

The 235/70R17 has a lower load capacity and that completely eliminates it in my book.

The 255/70R17 is larger in diameter by over 3% - outside the tolerance.  That would be the better of the 2 poor choices.