Not sure what would make you think that. Weeden and Tannehill's QB ratings last year were not significantly different from each other, and despite Tannehill's high QB rating in the preseason, Weeden's was slightly higher.
I don't think there's any reason to think Weeden will improve any less than Tannehill this year, and that my just start tomorrow.
I dont think tannehill is better than weeden... I know he is