A quick way to guesstimate the RPM change is to swap the numbers. 25" to 30", change to 30 and 25, so where you were turning 30 or 3000 RPM, now you'll turn 25 or 2500 RPM. No rear end ratios or OD ratios needed! It's a simple straight line graph, so 1250 RPM (half of 2500) was 1500 RPM (half of 3000).