Crankshaft underdrive pulleys that make a slight difference on V8s are considerably smaller than OEM and although they're benificial for some performance gains, most racing or hot-rod (not everyday type cars), there's often other problems that arise due to the slower driven accessory pulleys. If you can find a custom made pulley, you might then find that your PS isn't good at slow speeds, your AC only blows cold over idle, and/or your alternator doesn't charge at all idling. I have a hot-rod type, Ford powered car (nothing's stock) that with my March 50% underdrive crank pulley installed, my nifty chrome 100 amp alternator doesn't charge at all idling and that's probably not too bad in the day but at night, the battery discharges so fast that it becomes a problem. I'm even using a smaller overdrive alternator pulley, doesn't help at night. Some then say that going with a higher amp alternator (130a) might help but there goes yet another non-returnable $300 IF I want to try that.
Underdrives are best for racing where accessories or electrical draws aren't an issue, not always ideal for everyday driving.