Do you know how to read a tire size? More importantly, do you know how to convert it?

245/45r17 245mm wide, 45% of the width is equal to the sidewall height, 17 inch wheel.

so 245 times .45 times 2 all divided by 25.4, added to 17 gives you the diameter of the tire, which is what makes your speedometer innacurate (as well as the odometer).

[(245*.45*2)/25.4]+17 = 25.68" diameter 9.65" width
[(275*.4*2)/25.4]+17 = 25.66" diameter 10.8" width

which is why the 275 is a + size and it doesn't change the speedo much at all. The 255 sizes would have more of an effect.

275/50r15 is 25.83" diameter
295/45r15 is 25.45" diameter