Originally Posted by FBinNY

The fundamental rule for chain drive applies. Larger sprockets ALWAYS produce more chain life. So if a similar gear can be produced using the inner ring and a particular sprocket in the rear, or the outer ring and a larger one, opt for the outer/larger combination.

The reason is simple, larger sprockets mean lower chain load to transmit the same torque. Also the chain flexes less winding onto larger sprockets (chain flex = 360°/no. teeth), so you have less movement under lighter tension which is a win/win as far as the chain goes. There's also a subtle effect in that there's less chordal action with larger sprockets, which lowers tension spikes.

IMO sprocket condition (within reason) has little impact on chain life since the wear is occurring within the chain as it flexes onto and off the sprocket one link at a time. OTOH chain wear has a large effect on sprocket life because a worn chain doesn't engage smoothly and the engaging roller rubs down the teeth rather than sweeping cleanly into the trough.

If you've ever ridden a ferry, picture the difference between a captain who steers straight down the middle of the slip, and one who misses to one side slightly and rubs the boat down the side of the slip. That's the difference in how new and worn chains engage the sprocket.

So mix sprockets or not, it won't affect chain life, but it will affect cassette life, because the cassettes life is determined by the one most worn sprocket, and not by the average wear

BTW- you bought and paid for gearing to make riding easier. Use the gear that's best for you. Changing how you ride to preserve the chain is backward thinking. Reasonable chain care will give you the best chain life without having to make any other adjustments.