South Plains Wireless charges a user fee of $35 each month and $9.95 per gigabyte of data used. Tumbleweed Cellular charges a user fee of $15 each month and $13.95 per gigabyte of data used. How many gigabytes does a person need to use to make the plans equal? South Plains Wireless charges a user fee of $35 each month and $9.95 per gigabyte of data used. Tumbleweed Cellular charges a user fee of $15 each month and $13.95 per gigabyte of data used. How many gigabytes does a person need to use to make the plans equal?