Zack's right about the 4% size difference and the effect on your speedometer (and odometer). If your speedo was exactly dead-on with your original tires, then when you are doing an actual 60MPH with the new ones, the speedo will show 62.4. Similarly, an exact 100miles covered will show 104 on the odo. And the engine will be turning 4% more RPM's than before at a given road speed.

Main thing, though, is the load carrying capacity...check the sidewall ratings for max load and compare to your original tires. It will be less, but by how much? Then keep that in mind when loading up the old brick.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar 77 MGB, and a few old motorcycles)