tire size clarification please

[Synn]

37x13.5 inch tire without any issues? Meaning Would I lose top end on Highway ? What back spaces would help ?Its a
stock H2. no lift with no air ride .....If I go with 37x12.5 instead would I notice a difference in power like would she become a pig on a Hill going up ?

Thanks i looked aswell and have not found answers to all ?'s