Why do you want to mix and match in the first place? Are the tyres on the 19's worn out?
Running a 225 up front and a 245 on the rear is going to increase understeer,and increase rear grip and traction compared to running the same size all 'round.
running the 225/50/R16's on the front and the 245/35/R19's on the rear will feel odd. the front will feel sloppy and less communicative,and it's likely to understeer more,with slower response to transition.
If it was my car I'd swap to the 16's all 'round. the handling will be more predictable with similar profiles front and rear.
Justin...
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