14' vs 15" wheels

15's are an inch bigger. If there a little wider they will also put more rubber on the ground. What you have to remember is when the truck was built it was designed with the 14's they are designed this way for gear ratio all balanced together. Engine, trans, rearend, tires. If it is an automatic it will affect how overdrive shifts and you also may use a little more fuel but as far as it hurting anything not likely(NOS) I sure would check my fuel milage a few times. And I read a little better that you were down shifting so that is a 5 speed gear box and 15"inch will make you down shift more going up hills I had an 88 ranger with 5 speed and put 15"inch tires on it cause I was hauling wood with it most time I had to run in fourth gear with a load. But that truck sure could haul a load of wood and pull a trailer loaded with wood also. So I really wouldn't worry too much about it just enjoy your gift.(NOS) 8)