Tires: tire size -- trust the placard?, tire placard, odometer readings

Question
I have a 1998 Jeep Cherokee Limited, 4dr, 4WD.

My current tires are 225/70r15, and if I lookup my exact model/trim on tire dealer websites, they all end up with either 225/70r15 or 225/75/r15.

BUT...

I decided to actually look at the tire placard on the door -- and it reads 215/75/r15.

The tires currently on the car I purchase 5+ years ago, the size was selected by the tire dealer, and may or may not be the same size as when I purchased the car, used, in 2000.

So when I go to purchase new tires, I am wondering if I should trust the placard, or trust the database of tire-specs that tire dealers use?

Is it possible or likely that a few 98 Jeep Cherokee Limiteds were made with different tire/wheel specs, and these exceptions never made it into the databases?

Or could the placard be wrong -- is that at all likely to happen?

Will the size difference matter performance-wise?  How much?

By my calculation it is about a .7% difference in overall diameter, and so a .7% difference in speedometer and odometer readings.  

I also noticed that the correct (according to placard) size tires are significantly cheaper, at least for some brands/models of tire.  Any particular reason why that would be?

Thanks.

Answer
Scott,

Vehicle manufacturers are required by law to apply the placard, so you can trust that your vehicle originally came with the size indicated.  There are reference books that tire dealers use to find out what tire size originally came on vehicles, but there are errors in these books.  Sometimes, the books are "updated" to show what are acceptable substitutes - and I suspect that is the case as 75 series tires have pretty scarce and a 225/70R15 is reasonably close to a 215/75R15 in virtually all parameters.