anyone know if the jd 770 is the same as the 790 on ratios? My brotherinlaw has a790 with r4 tires rear 15-19.5 front 25-8.50-14 my new to me 770 with 300hrs has lawn tires 13.6-16 and 25-8.50-14 i would like to find if they would work on my if so where i could to find them or ag tires thank you