newb tire question for 1st gen taco

I recently bought a 99 taco prerunner that had a slew of problems and now im at tires and now i have another possible issue. On the door jam the stock wheels were 225/75R15 but the tires it has on it now are 245/75R16. everything else seems to be stock even the rims are Toyota rims but the tire size according to my research is not stock what could these rims have come from. Also im looking for a decent set of tires for cheap but looking to keep the rims since i cant afford a new set. My question is what would be the difference if i were to get 225/75R16 since they are significantly cheaper. Also what other problems could i get from these bigger tires besides gas mileage.