OEM sprockets are 15/38. If you go to a 15/42 combo like many of us have, you will get much better low end performance. You can take off from a stop on steep hills without that awful groaning sound from your clutch. You will also rediscover your 6th gear at legal (U.S.) freeway speeds, as the 15/42 combo will put you in the 4,000-5,000 rpm range at most US legal highway speeds.
Going to a 42T rear sprocket will require you to add an extra link or two to the chain. You may want to wait until your next sprocket/chain change depending on how much life you have left on your current chain. My chain was fairly new when I had the 42T rear sprocket added, so Ducati Seattle just added two new links.
I see that you live in Illinois. If most of your riding is Midwest flatlands, you may want to initially try a 15/40 combo, which may not require the extra links in your chain. I had a 15/40 combo on my 02-ST4s, but I much prefer the 15/42 combo for our hills out here in the PNW.
I went with the 15/40 at the same time I did the first tire change. My chain was obviously still good and the 40 tooth rear sprocket did not require a new chain or any additional links. It is much better than stock but I still find the gearing a little bit too tall. I will go to a 42 tooth rear sprocket and get a new chain on my next tire change.
When my ST4s still had less then 500 mi on the odometer, I swapped the front 15 tooth sprocket for a 14 tooth. That change made a big improvement in low speed drivability. Much less clutch slipping needed from a start, and 2nd gear was usable for slow speed turns in town without needing to slip the clutch. A bit later I put a 40 tooth on the rear and that has made 6th gear usable at normal highway speeds. Comes time for a new chain I plan to reinstall the original 15 tooth on the front and go to a 43 on the rear. As the others have already stated, changing the gearing on this bike is probably the single best change I’ve made.
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