Originally Posted by HillRider

As noted, it's simple arithmetic. As DaveSSS calculated, 39/34 = 1.15 so switching to the compact will make the pedals 15% easier to push. You said you have a 53/38. Is that a typo or miscount? 38T chainrings are available but they aren't standard on any OEM road crank I know of but if it's correct the compact will lower you low gear by only 11%.

You also said you have a 12x25 cassette. Going to a 12x27 will lower the low gear by another 8% and the only cogs that differ are the largest two. A 12x25 is 12......23,25 and a 12x27 is 12......24,27. All the other cogs are identical.