You probably know this, but just in case, the 40 and 45 numbers are a percentage of the width (295 mm vs 275 mm) so a 295/40R20 is approximately 29.3" tall while a 275/45R20 is approximately 29.75" tall. A 295/45R20 tire is about 30.45" tall.
Check this out...
http://www.rimsntires.com/specs.jsp