Here's a question. Because of flat replacements, I am currently in need of two (not four) new tires for my wife's daily (Prius V). My preferred tire for this application is the Pirelli P7 series (with all the treadwear!). I realized yesterday that I'm already running what Pirelli would call a mismatched set, meaning, I have three P7 "Plus" and one P7 Plus II. This has been fine. The Plus II has been discontinued in favor of the Plus 3, so the question is....do I spend $380 on two Plus 3's and run three generations of the same tire at once, or do I bow to the, "We do not recommend mismatched tires" I got from Pirelli support and spend $615 on four? I would be less inclined to mismatch brands or even different product lines within the same brand, but the fact that this mismatch would be within the same product line (just a newer generation), combined with my just-fine experience with mixing Plus and the Plus II, the single tear that would drop from my eye if I got rid of two perfectly good tires, and the $200 I'd rather use for something else are quite strong arguments for just getting two.
I appreciate your thoughts!
The problem you are going into is the two new tires need to go on the back, many installers and all manufacturers will insist on this, but then you will end up with two different tires on the front which isn't desirable.
Personally, considering its a Prius, I would put the new ones on the front, and go with the older tires on the back, even if I had to do the swap myself.
In reply to jharry3 :
Good point. Also, for whatever reason, it completely skipped my mind that I could sell the good takeoffs....
jharry3 said:
The problem you are going into is the two new tires need to go on the back, many installers and all manufacturers will insist on this, but then you will end up with two different tires on the front which isn't desirable.
Personally, considering its a Prius, I would put the new ones on the front, and go with the older tires on the back, even if I had to do the swap myself.
This. Let them put the new ones on the back and then swap them front-to-back yourself, if the shop zips the lugs up to 200+ft-lbs as many do, you can put them back on at a reasonable torque value while you're at it 
If the tires are just slightly different variants/generations of the same model, that's about as close to running identical tires as you can get and I wouldn't worry about mismatching so much as tread depth and tire age.
If it was awd I would say problem. But on that car no problem. Currently running 195 on the back and 185 up front, just because I got an unfixable hole in the back left. No warning lights, no problems. I love the way prius handles but honestly not like you will be getting into the oversteer/understeer speeds.
Assuming they're all the same size, the situation you've described does not meet my personal threshold for "mismatched" . Also, as others have noted, I'd totally put the new ones on the front and keep them there for quite a while until front/rear treadwear is as close to equal as you can get without other bad wear issues like bald edges or cupping. And make sure the older tires are less then 6 years old.
Personally for the price of 2 Pirellis i would buy 4 Goodyear Assurance from Sam's Club.
I used to worry about this, but our work cars get whatever the cheapest tire possible is on a given day replaced one at a time. I don't believe any of them have four of the same brand and they seem to work reasonably well.
