How do you rotate tires when the fronts (225/45-19) are a different size from the back (245/40-19)?

 

Unless you have a square setup, which it sounds like you don't, you can't

 

The only thing you can do is rotate them side-by-side if the tires are asymmetrical. If they're unidirectional you'll have to dismount the tires.

 

Thanks,
That's what I thought

 

If directional, side to side with dismounting them from the wheels and properly remounting them for the opposite side. If asymmetrical, entire wheel/tire can be moved left/right. Just can go front/back. Anything you can do will help tire wear.