Underdrive pulley????

[BLK1500]

What are some benifits of an underdrive pulley, and I know that there smaller than stock, what size of a belt should I get if I get one. and does it really help on my gas

[gmctrucks]

I have one on the harmonic pulley and the other on alternator what I seen is a lot more power on my 93 F/S 5.7 GMC Ext-Cab.

[Chevy_Finatic79]

why do you only have 2 pulley's GMC? dont you need to purchase a whole set of matched size pulley's to get the real benefit? I have never seen these underdrive pulley's so I'm kinda curious

[BLK1500]

I was WOndering since you have a 5.7 and both our motor are alike, what size belt should I use, since the pulleys are smaller

[gmctrucks]

The set I bought didn't come with a water pump pulley and when you buy the pulleys they will let you know what size belt to use I don't remember what size I have on it also im running a 160 stat I know that when I bought them there were many of companys offering just the two pulley set I remember before I placed them on my truck my dad and I test drove my truck from a dead stop then we tested it afterwards we were both shocked it burned out all of first gear then it hit second gear and that's was without a shift kit buy the set your going to like them. My truck is a Ext-Cab with a lot of extras leather/wood 10'' wheels running boards and so on and I average about 17.8 mpg on flat land and in the hill country it goes down to 15 mpg.

[Moostang]

I wouldn't trust what they say for belt size.. it may get you close, but doesn't always work. I know the mustang people I knew all seemed to vary in belt size for some reason, and our shortbelts we ran at the track also seem to vary. They usually could swap.. but not something I would trust for daily driving.

I think underdrive pulley were supposed to get you upto 15 extra horsepower.

[gmctrucks]

Moostang I know that I went with a different size because I didn't like they way it fit the tensionor just didn't seem right I know that I can get the belt size but I would do like Moostang says and try some first I remember now what happen it was the belt they said to buy it was going to leave the pulley to close to I think it was my water pump pulley so I went with a smart one now if I would have kept it and if the belt would have strectched in time it would have hit it and what I did since my truck is a ext-cab I removed the rear armrest pocket dealy and placed the number of the belt in there and in time while I have replaced the belt I place the old one in there.

[heavychevy85]

do the mpgs increase at all or not with the under drive pulleys

[gmctrucks]

I really can't say but your going to like the power it gives you.