Gear ratio defines vehicles torque and power delivery behavior. When tyre is upsized / size changed the effective radius changes which results change in effective gear ratio.
Example: Original size tyre of 205/55R16 if replaced with 215/50R16, this result in higher tyre dia which results in higher torque delivery. Effectively lower gear ratio.
If Original gear ratio is 3.55 , with 215/50R17 this will change to 3.47.