I easily get low to mid 20's in the city in my LS. That is, unless I have fun.

Im around 19 also in the city strictly, driving spirited and launching from lights constantly lol.

Guys, this an old thread that just got necro'ed for this following question. :

In my experience with my Camaro's, yes, there is difference between what the computer says and hand calcs. Hand calculation will always be the most accurate.

You are incorrect because you can never be assured you get the tank filled to the same level every time, also different pumps will measure slightly different, have different cutoff points, if you are setting level or slightly uphill or downhill also temperature changes from the times you fill can effect how much you can get in the tank. There are so many variables you cannot take into account when manually calculating fuel mileage.
Anyway I have never found mine to ever be off by more than 1/2 mpg.

That is surprisingly low. My OnStar shows 20 mpg for an LS3.

Sorry, I guess I should emphasize that it is in my experience, filling up at the same gas station, at the same time of day (before work), placing the pump handle into the car in the same way, (with the first part of the pump's spring over the lip of the receptacle), that my computer has been off by 1-2 mpg before. Usually 1 mpg. Everything you said is valid, of course.

Thats what I get when I'm driving around town.

I have a buddy who bought a 2010 1LT RS, new. He's a Chevy guy and I assumed that he would be happy with it, so I was shocked when he told me he was going to get rid of it. I asked him why and he told me that it was a gas hog. He said that he was lucky to get over 20mpg on the highway. When I told him that I could get 26-27 on my LS3 at 60mph, he said he wished he'd gotten a SS. Of course told him that I could get as bad as 12-13 in town, driving aggressively. He sold the car shortly thereafter.
I have another buddy with an '08 Corvette, LS3, auto, 2.56 gears. He claims he can get up to 35mpg at 60-65mph and he says he almost always gets over 30mpg on road trips. His car ran a 12:50 at the dragstrip, so it's no slouch. I figure that's the beauty of a lightweight, aerodynamic car with a high torque engine, extremely tall rear gears and a double overdrive transmission.

25-27 on the highway in my LS3...city though...12 ish..

That's about exactly what mine does. It must be a RJT LS3 thing.

Speed does effect the mpgs on these cars. They like to breathe and you can actually get better hwy mpgs running 65-75 than at 50. The higher rpms allows the engine to haul 2 tons more easily than lower rpms at lower speeds. Also, driving at 20-30 mph with frequent stops kills mpgs.