Speedometer off?

It will be about 3 MPH slow. Bigger tires less tire revolutions.

Depends on the aspect ratio vs the width vs the wheel diameter of the tire. For instance a 245/40-17 has an aspect ratio of 40 meaning the sidewall is 40% of the width in height for each sidewall. So the 245/40-17 has a total height of around 24.7" as shown below. Now the height of a 205/60-15 rounded off to the nearest tenth of an inch would be identical in height as the 245/40-17.

Do the math:
245/40-17 tire:
245mm X 40% = 98mm for each side wall height.
2 sidewalls = 196mm.
Convert the 196mm to inches: 196mm divided by 25.4 (conversion factor from mm to inches) = 7.7 inches.
Add the wheel diameter of 17 inches + the 2 sidewalls of 7.7 inches = 24.7 inches total tire height.

205/60-15 tire:
205mm X 60% = 123mm for each side wall height.
2 sidewalls = 246mm.
Convert the 246mm to inches: 246mm divided by 25.4 (conversion factor from mm to inches) = 9.7 inches.
Add the wheel diameter of 15 inches + the 2 sidewalls of 9.7 inches = 24.7 inches total tire height.