The 7.5" x 2" Front & 12.5" Rear Pnuematic Knobby Tires can be ordered as an option to help maneuver over rough terrain by providing a bigger surface area. This is the UPGRADE price for this item at the time you purchase a Convaid Pushchair. If you need replacement parts, please call us at 1-800-371-2778 or send an email to info@adaptivemall.com to find out replacement part prices. You must provide the Serial Number for your existing Convaid Push Chair when ordering accessories to add to the Convaid Push Chair that you already own. Please enter the Serial Number into the “Comments Section" during the check-out process.