I usually like to see about 3/8" max difference. When I've had sets that didnt match I usually air the smaller tire up to around 30 psi for about 15 minutes. If you can set it out in the warm sunshine it will help. There have been times I left them aired up for a longer time. Basically your just trying to stretch them out a bit. I guess you could send them back but I've always streched them and they worked fine.