The equation for gear inches = (Front ring/rear cog)x wheel size. Wheel size is in inches. Do a bit of algebra and solve for Front ring= (gear inch/wheel size) x rear cog. Assuming a 700C wheel, your existing gear, g.i. = (48/34) x 27 = 38 gear inches. For the near low gear of 25 teeth, the ring needed would be front ring = (38 gi/27”) x 25= 35 teeth. If you don’t want to do the math, you can use Gear-Calculator site. Here’s the same calculation I did in graphic form. You can see that dropping to a 35 tooth gear, your high gear is going to suffer significantly. You are going to spin out fairly soon and spend a lot of time coasting.