Underdrive pulley, new belt or what

I got a 1.65pound underdrive pulley. It says to replace belt, does that have to be done?

Yes, underdrive pulleys are smaller. I could help you out a little more if I knew what motor was in your car.

420a

p/s, a/c belt
Bando- 5pk1270
gates- k050500
dayco- 5050500
goodyear- 4050500
day/pl- 500k5
Alternator belt
bando- 3pk800
gates- k040315
dayco- 5040315
goodyear- 4040315
dayco/pl- 315k4