Why does Gm use too small of opening in the fuel nozzle? It gets frustrating when the pumps are the large type only.

Federal regulations. OTR HDT's buy IFTA fuel (Intl. Fuel Tax Agreement) at truck stops. These are the "large" 30mm nozles you are refering to. Although older stations may not have switched to the 20mm non-IFTA nozles yet, they will have to prior recertification for 2007. GM has reduced the filler neck size to meet that same regulation (and save you 3 cents tax per gallon).

My '05 came with an unrestricted one - you could almost drop a golfball down into the tank.

They didn't reduce it, they went bigger. On the older trucks it was smaller than it is on the newer trucks. My 04 has one big enough to fill from any nozzles I have found.

Way back when we had the OLDs 98 diesel the smaller opening on the tank was to prevent people from absent mindly putting gas in instead of diesel. It saved my butt more than once.Almost did it on my 03 gmc to. Stopped in to tank up one night and was fiddling with pump nozzel, try to get it into the tank when a pretty young lady on the other side of the pump island said " I didn't know you cculd put gas in a diesel truck ". My wife about cracked up hearing that and later told me I wouldn't have done it if I'd been paying attention instead of flirting with the girl. She never lets me forget it either.If she's with me when I fuel up, I always hear " Don't forget, Green Nozzle "

Federal regulations. OTR HDT's buy IFTA fuel (Intl. Fuel Tax Agreement) at truck stops. These are the "large" 30mm nozles you are refering to. Although older stations may not have switched to the 20mm non-IFTA nozles yet, they will have to prior recertification for 2007. GM has reduced the filler neck size to meet that same regulation (and save you 3 cents tax per gallon).
I'm not so sure about the regulations. . . do you mean on the truck or on the pump? I have no problem fitting the big pumps in my truck. . . Any clarification appreciated. . . Thanks,

Dennis