Ive gotten the new G8 over 20mpg in the city. 23.076 to be exact. Of course this is doing everything I can to keep it in 6th gear in manual override and feathering the throttle.
2009 Civic Si 21/29 MT. Yes you can feather it and get more but how much more in a
manual transmission? Won't know until someone tries. I'll be more than happy to if I can find one to drive for the full tank of gas on a test drive. So we have:
2.0L 197 hp 21/29 MT Honda Civic Si
6.0L 361 hp 15/24 Manimatic Pontiac G8 GT

Link to someone who got just about the same milage I got using the G8 GT on a
magazine write up:
Pontiac G8 GT MPG Review

Link to Yahoo Cars MPG write up on the 2009 Civic Si
2009 Honda Civic Si Sedan | New Honda Sedans, Sports Cars - Yahoo! Autos

G8 GT Msrp : $32-35k (Sticker off our new car, fully loaded)
Civic Si Mrsp : $24,105 (Yahoo Cars, fully loaded most same options)

So I managed to take a V8 and get better gas milage than a Honda Civic Si
ALL CITY DRIVING.

Pontiac G8 GT 1/4 Mile time : 13.74 (Dragtimes.com)
Honda Civic Si 1/4 Mile time : 15.22 (dragtimes.com)

So you have $10,000 +/- to get the Si to a 13.74. Not hard, but to also make it
completely reliable in any condition for the long haul, I think it will come close
to having the same price tag.

And no I'm no V8 lover. My favorite car ever is my best friend Jay's 2001 Acura
Integra Type-R That has an AVERAGE win rate over the new z06. It's also reliable,
will start every time no questions asked, and is in full trim will full factory options
(no gutting). He also has to run 93 octane (92 makes the motor knock) and the total
build cost is shy of $5,000 the price of a new z06. Gas milage? try 12/18.