70/65-6.5 is
70mm wide with a 65mm sidewall for a 6.5 inch rim. So effectively that is a 11×2.5 (11inch diameter x 2.5inch wide)
As an example a 80/65-6 would be 80mm wide, 65mm side wall, to fit a 6 inch rim. Effectively making it a 10×3 or 10 inch diameter with a 3 inch width.