Put your old tire size and your new tire size into the "calculator" on this site and it will tell you how much the speedometer will be off.
https://www.tacomaworld.com/tirecalc
Your odometer will be off also if you track your gas mileage. Then just keep a close eye on the speedometer while you're driving. Much easier and cheaper than changing the speedometer gear. Not that it's a very hard job or anything but this way is totally free.
I went from a 14" stock size on my '97 Ranger to the closest size I could find in the tire I wanted and my speedometer was only off by 5%. At 60 mph it read 63. At 30 you wouldn't even notice a change. (31.5 mph)