Tires: Mixing Tires, tread depth, tire sizes

Question
QUESTION: I have 4 P245/75 R16 tires on my full size pickup truck. One is much more worn than the other three. I would like to change all four at once, but the older one cannot wait. Can I put a used P245/70 R16 tire, with about the same tread depth, in its place for a few months while I get the rest of the wear out of the three newer tires?

ANSWER: Lance,

This is just not a good idea.

Vehicle tend to pivot around odd tires in emergency maneuvers.  The more different the tire, the more the tendency.  Changing tire sizes is a large difference.  This makes this change a safety issue.

Besides, if we are only talking a few months, then the issue boils down to the risk you are taking compared to the money you are saving.  Sounds to me like you would be on the wrong end of the equation.



---------- FOLLOW-UP ----------

QUESTION: If the tires were new I would agree, but they are used. It cannot be any worse than driving on one bald tire and three tires with decent tread on them.

I average 125 miles per day that's why it will only be a few months. The bald tire has now gone flat and the spare is the same size but a different design and is cracking. Which is more unsafe?

The idea is to get back to 4 equal tires and I cannot afford to throw away perfectly good tires. Tires in that size are hard to come by used. If I buy a new tire I just continue the problem and the vehicle will still tend to pivot around the odd tire in emergency maneuvers.  The more different the tire, the more the tendency. A new tire would even be a bigger difference and even more unsafe or am I missing something?

Answer
Lance,

Yes, you are missing something.

The cost of repairing a drivetrain.  If tires are different diameter, then the drivetrain runs at different rates, and the center differential has to compensate.  Too much of a difference, and the center differential will burn up.

What you are talking about is a whole size change.  And it's the miles you will put on in those few months that make a difference.

And while you seem to think that you can not afford a couple of hundred dollars to buy 2 more tires, the question is:  Are you prepared to pay thousands of dollars to repair a drivetrain?  That's what the risk is.

AND you can use a P255/70R16 in place of a P245/75R16 - and there are tons of those available.