If you visit Tire Rack's website (or any other tire seller), they'll explain the sizes and what all the numbers mean. In short, the difference in those 2 sizes is that one has a sidewall height that is 45% of the 245mm section width. The other sidewall is 40% of the 245mm width. Mathematically, that means that the "40" tire is 12.25mm less tall (on a radius, not diameter), or an overall 25mm (1 inch) in diameter.
You will be moving just a little slower than your speedometer registers. If you have a GPS you should be able to compare your speed and your speedo. At 60 on the speedo, you might be doing an actual speed of 57, 58, or 59.
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