15 is stock, together with 45 rear.
1 tooth front alters the rpm by about 500rpm. So going one tooth down gives you 500rpm extra at the same speed.
So if 15/45 gives you, lets say 95km/h at 4000rpms, changing the front sprocket to a 14, will set the rpms at 4500rpms to go the same speed.
You can play with the numbers at www.gearingcommander.com I use the 1990 xt600. I added that one, and its the same from 1990 and up to the last models.
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