Why would you want to do that? The difference is 3 gear inches between a 25 and a 27, which under most conditions is going to be completely imperceptible.
The difference in gearing is fairly great with small cogs. For example, the difference in gearing between a 12 and 13 cog is about ten gear inches, but once you are on cogs that have teeth in the 20s, the difference in gear inches between cogs that have one tooth difference goes down progressively, so that the difference between say a 27 and 28 is one or two gear inches.
So to get a ten inch gear difference from your 25 (which will feel just like the difference between a 13 and 12) you would have to get a 32/33 tooth cog.
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Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1